There are two kinds of people that buy android phones:
People who want something different than the iphone:
If this is you all you need to do is buy any android device that comes with a current version of android. this It will be either 2.2 (froyo) or 2.1 (eclair). Do not buy a phone if it comes with any version lower than this because you will be missing out on some key features. Other than that, just keep in mind that like the iPhone navigation doesn't work in the middle east (you need separate apps for it) and not all software is available from the Android app store (called the market).
People who like to back around with their phone "jailbreaking" and "rooting" it:
The thing you have to keep in mind with android is that very quickly you will want to start modifying (hacking) your phone to install the latest version of android available. To do this, it is always best to buy one of the Google developer phones - like the Nexus One. These are guaranteed to be unlocked and allow the most modifications. They also come with the default Android interface (called the "google experience") which included the android market, gmail, navigation/maps et. al. Most other phones come with customized interfaces. HTC has it's own interface, called "Sense UI"; Samasung has their own and so does Motorola "Moto Blur".
Keep that in mind and you will be better able to decide which phone is better for you.