Operating system...following. Ford, Chevy...
iPhone started it all 3 yrs ago...Apple built a huge following. There are HUNDREDS of THOUSANDS of apps for the iPhone. That said, Apple has kept a lock on the hardware and software. Doing certain things just isn't possible without Jailbreaking the iPhone (removing Apple's restrictions).
Android-based (Droid, EVO, etc.) have more freedoms. In fact, Android sales are up almost 900% over last year (yes, that's not a typo). Android devices still have some restrictions, but no where nearly as many as iPhone. If you're considering a newer Android device and want to play around with it, look for one that allows you to "root" it. (same as jailbreaking). The Droid X finally has a root for it and PDAnet can be used.
There are advantages and disadvantages to each. I'd look at what your dominant carrier offers. For example, out Michelle's way, ATT coverage is non-existent. So whenever I'm out there (like I will be in 2 days), my iPhone will not have service - not until I drive 20-30 mins. So, Verizon is the way to go out there...that rules out iPhone...and why she has a Droid X.
If you're in a major Metro with coverage from most carriers...then compare 'em. Ford, Chevy...
I have an iPhone...because work gave it to me, prior to that I was all Blackberry. And if tomorrow we change to Android, that's what I'll have. Now, if I were paying for it? Not sure, but I may lean toward an Android...While their app store only has about 65,000 apps, it's growing every day. If Apple doesn't pull their head out, they may start losing...
My :2cents: