So...choosing a smart phone can be tough, because yes...there are allot of choices! When you choose your next smart phone, unless you buy off contract your next phone, you are deciding for the next 2 years, because you'll be locked in...in most cases.
Smartphones are really about the operating system and the hardware. It looks like allot of choices, but in actuallity, it's only 4 major operating systems..
Android (All your Motorola Droids, LG Revolution, HTC Thunderbolt, Samsung n Nexus, etc..)
iOS (iPhone only)
BlackBerry (All your BlackBerry devices)
Windows 7 (Nokia, HTC, and whoever else has started to pick it up..)
Android has now trumped iOS with 40% of the US market share, only because it's 1 iphone vs a bunch of various Android handsets on the market. Also has a huge ecosystem of apps and features. Android 4.0 is the latest/most appealing version, and available on the Samsung Nexus and most phones in January like the Droid Razr.
iPhone, well...it's the iPhone. It's got 28% of the US market share. It's on iOS 5 and has a neat little feature called Siri for a personal assistant. Huge ecosystem of apps, and easy to use. Great hardware.
BlackBerry is falling hard, yet they are trying to keep up...The Operating sytem is less appealing with less features, plus plagued with recent outages, unless they overhaul their OS, they're in for some more trouble.
Windows 7 is really just starting, growing slowly. More and more apps and is said to be very easy to use. It's based on a tile system for activity. I think it will grow more with windows 8 on both mobile and pcs.
There are some other operating systems like webOS (Palm, HP), Meebo..etc. I currently use a Palm Pre 2 which is webOS, and that's because I love the operating system, but it's a dying phone with discontinued hardware. I'm hoping it gets revived, otherwise I will need to make some choices myself here...
The other big factor to look at is 4G/LTE and how fast it's growing. If you are going to be with a phone for the next 2 years, do you want 4G? It's growing fast, especialy Verizon.
In summary...
1. Choose an operating system that you like most.
2. If Android, choose good hardware (Nexus or RAZR, etc). The iPhone has one choice and it's very good already.
3. If you want 4G, at this time it's mostly Android phones only. Some carriers have Windows 7 4G.
4. Read online reviews.