Here's the thumbnail sketch of the current smartphone environment:

OS: Your choices are the iPhone's OS (based on Mac OS X), Android, Windows Mobile, BlackBerry and Other (Maemo, Symbian/S60, WebOS, etc). Taking those one at a time:

iPhone: The iPhone has the advantage of a vast app catalog and the usual Apple spit-polish. In practical terms, that means that the default apps (browser, mail, maps, contacts, etc) work together reasonably well and it's a well-integrated package that can be easily extended with one of the 125,000+ apps. There aren't too many downsides to the OS itself, it's a well-conceived mobile OS that works well for most users. There are things I personally don't care for, both as a developer and user, but overall it's a strong OS. It does bother me that the app store is controlled by a sole entity (ignoring hacking your phone), but that's more of an intellectual rather than a practical objection.

Android: Built off of Linux to be a true open source alternative, Android's primary advantage as an OS is that different manufacturers are taking the ball and running with it. Some are doing nice things (HTC), some are fumbling around a bit (Motorola) and some are really quite lost in the wilderness (Samsung), but the competitive ecology out there matters. It's also the case that Google is moving the core OS forward very rapidly, with four major (and free) updates in 2009 alone. However, that core technology strength of openness doesn't always result in a completely coherent user experience yet. I have absolutely no problems getting around in Android, but some folks find it to be a less clean experience than the iPhone (I find the opposite, thanks to Apple's allergy to buttons). The app store has some catching up to do, with about 20,000 apps available, but it's an open store, which matters, and it's going to catch up. In the short term, pretty much every productivity category is reasonably well represented, and the shortfall of apps is primarily in the realm of games (and fart apps). That's not to say there aren't plenty of engaging games available, but it's a ways away from the iPhone support. Give it time, though: at current rates, the Android will pass the iPhone in terms of total mobile web usage in June, and that's the kind of thing developers notice. In terms of day-to-day usability, I find Android devices to be comparable to the iPhone.

Windows Mobile: Only on phones where the base Windows Mobile shell has been completely replaced (ala HTC's HD2) do you get a modern experience, and that phone is nice in spite of WM and not because of it. This may change once the mythical WM7 ships, but in the mean time, I don't think this is a reasonable contender unless you need its Exchange support, which is still the best of all the major OSes.

Blackberry: An OS that is showing its age, even the 5.0 build is difficult to get around in and shows signs of being built on older technology. You can adjust to it, but the learning curve is pretty steep. For pure messaging, this is still probably your best choice, but for other things (the web browser is not good), this is down on my list right now.

Other: WebOS (Palm Pre, Pixi) is nice, but until they get an app store rolling, I'm not sure it's a contender. Once that gets fixed, we'll see if they get support. Maemo (basically Debian Linux with a shell) could be fantastic for a certain segment of the market, but right now, it's still in its infancy. I'd love to see it succeed, though, and find a US home. Symbian/S60 is still pretty dominant in Europe, and I carried an S60 phone for a couple years, but it's iffy as a choice in the US market.

HARDWARE: The iPhone has essentially one choice, there are a bunch of Android devices out, there are an assortment of Blackberries and WM devices, so this can get confusing. At this point, the iPhone is no longer cutting edge. Yes, it's pretty fast, but the lower resolution screen compared to some of its competitors means it's not the baddest ass hardware on the planet. Here's where the Droid, Nexus One, HD2 and some others shine: that 800 pixel resolution in landscape mode makes a huge difference in web browsing. Because I like having a physical keyboard, the Droid is my easy choice for nicest phone hardware out there (and the keyboard is not awesome, but it's perfectly usable), but if you don't care about a keyboard, the Nexus One is superb. I believe all on-screen keyboards suck, but some suck a little less than others (and Apple's is clearly the best of the bunch), but the Android ones are usable. As far as Blackberries, RIM makes nice, solid units and still is the champ as far as keyboards go, which is why they're still the kings of messaging. The 9700 is probably the best one out there right now.

NETWORK: There are two groups of carriers in the US, the GSM carriers (AT&T, T-Mobile) and the CDMA carriers (Verizon, Sprint). The major differences are that CDMA is pretty much a North American technology (yes, it can be found in some other places, but bear with me) while GSM is available worldwide and that the 3G network for the CDMA carriers doesn't really allow simultaneous voice and data. Sounds bad for CDMA, right? Well, in practice, neither of those make much difference. If you're going to roam internationally a lot, OK, but if you're going to just take the occasional trip overseas, just get a prepaid phone when you get there and chuck it when you're done. It's way, way cheaper than adding international roaming to your existing plan, and international data prices are ludicrous when added to a US plan. As far as the data and voice thing goes, phones are good at deferring your data traffic for incoming calls, so all that really means is you can't look something up on the web while you're on the phone. Whoopty shit. You can still look at, say, your contacts and you can always use the magic phrase "let me call you back in five". In practical terms, it really doesn't make a difference.

Now, as to the specific carriers, you can't really use phones from one carrier on another network to full ability. Why? Verizon and Sprint keep their networks pretty well locked down and AT&T and T-Mobile use different frequencies for their 3G (high-speed) data networks. You can use a, say, Nexus One on AT&T, but it'll be poky. As far as quality, Verizon and Sprint have the best built out networks, by virtue of getting started on them earlier and continuing to invest. AT&T's network is overstretched and is pretty poor at this point, and T-Mobile's network is fantastically unreliable, especially in the Seattle area.

Conclusion: If I were buying a new smartphone today, I'd ask myself the following questions:

1) Am I using this primarily for messaging, and almost nothing else? If yes, I'd get a Blackberry, either the 9700 (if T-Mobile's coverage was good in the areas I lived and worked) or the Tour from Verizonn.

2) Is awesome Exchange support a must have? If yes, I'd hold my nose and get an HTC WinMobile device, either the HD2 or the Touch Pro, probably.

3) Is the best selection of games at the moment important? Am I going to be replacing a media player at the same time? If yes, buy an iPhone. I think AT&T's network is poor and you might kick yourself after the likely summer refresh of the hardware, but the iPhone's biggest strength at the moment is apps.

4) Is a physical keyboard important to me? If yes, get a Droid. It's what I have, and it's excellent.

5) Do I really need it now-ish? If yes, hang on for just a bit and get a Nexus One, as long as T-Mobile is acceptable where you live and work.

6) If you made it this far, my best guess is that you should wait for the Verizon version of the Nexus One.

Hope this helps.