To get a sense of what android is head back to the mobile phone wiki and read the article "General Android Overview." This can then be used to describe phones that run android and their good/bad points.
HTC Hero (G2 touch)2.1 (Eclair) or 2.2 (Froyo). The handset is in the process of being upgraded to android 2.1 but HTC have said that it will not see an official build of android 2.2 (Froyo).
You can read this Engadget review and to a certain extent I would agree with the conclusion. In terms of hardware the phone was released with the A11 arm processor, the same processor that was in the iPhone 3G. At around the same time the iPhone 3GS was released with the arm Cortex A8, the same processor which is in the Milestone/Droid. This lack of recent hardware is coming back to haunt the device and as such it does not officially support some of the newer "bling" things such as live wallpapers or flash 10.1/ (Froyo).
Many see this phone as the successor to the Hero. It features a similar form factor (the chin) but is encased in a uni-body aluminium casing giving great aesthetic appeal. However, it also improves on many of the features of the Hero with a faster processor, much improved screen and a flash for the 5MP camera.
This phone is HTCs release of the Nexus One. Released with Android 2.1 (Eclair). This is the most powerful HTC Android currently available in the UK. Featuring the 1GHz Snapdragon processor from Qualcomm it it far smoother than the previous HTC models. It also sports a 3.7 inch AMOLED screen which provides crisp graphics and an impressive multimedia experience.
Google Nexus One
Designed and manufactured by HTC, this phone is Google branded and comes with vanila android, i.e. no modifications. For these two reasons it means that the N1 has already received it's official 2.2 (Froyo) build.
Milestone (Droid)2.1 (Eclair), it has however been updated and should be seeing 2.2 (Froyo) in the near future. The Milestone, or Droid in the US, is one of the few Android devices to have featured a hardware keyboard since the launch of the flagship G1 and so was met with instant appeal from certain users. Although the Droid (and Verizons "DROID" branding in general) took off in America, the Milestone received much less attention in Europe and was nowhere near as successful as it could have been. This is still one of the most powerful Android devices available and, although it's aesthetics leave a lot to be desired, it is a good choice for anyone who depends on or prefers a hardware keyboard.
Samsung Galaxy S
This is Samsung's flagship handset and uses there exclusive Super AMOLED screen technology. The handset currently runs on 2.1 (Eclair) with a version of Samsung's Touchwiz User Interface placed on top. This handset features a similar processor to the snapdragon called the Hummingbird it is also clocked at 1Ghz. The Screen used in the phone is 480 x 800 pixels and 4.0 inches in size and is perhaps the most impressive feature, as the Super AMOLED screen technology is alot more clearer in direct sunlight than the older AMOLED version.
Other Buying Advice
Due to android being always connected to the internet to syncronise itself (Contacts, calendar etc) it is recommended to get an android phone with at least 500MB data allowance a month