If I were a t-mobile customer, I'd deff. get the Nexus one.
I'm a VzW customer and got the Moto Droid for Christmas. From what I've read, the Nexus one will have next gen hardware as compared to the Moto Droid, which at this point is pretty cutting edge. Both Moto Droid and Nexus one hardware will support the newest Android OS (2.1), support up coming Google earth, updated GUI, and more.
Nexus one is a soft-keyboard only phone, where the Droid does have a physical + the option of vert+horizontal soft-keyboard, so that is a big difference. Both great devices.
If you are heavily invested (and tied to) iTunes, seems like someone would have a strong case for staying iPhone. There are android apps to manage your iTunes, though.
I would say the apple apps are more developed, but that platform is closed, where as android is not, but the app quality is not as consistent. Still, there are 1,000's of GOOD aps that enable your android device to do all sorts of amazing things.
Things I enjoy doing on my Droid (all the blow FREE):
- Barcode scanning and price shopping, while out
- Using Googles to figure out what I'm looking at, or find a phone number of a business
- Playing NES, SNES, SEGA etc. ROMS (which you can't do with apple)
- root access and command line access to the linux based OS for total control
- Customizable work space panes (up to 7) with weather, tools, and other widgets (not just shortcut icon's to apps)
- taking DVD video, clips
- Ease of file transfer with FTPServer running on phone
- Use phone as FTP client
- Easy SD Card file management, from anywhere, any PC (no iTunes required)
- interchangeable SD Card (great for loading high quality movies)
- 2.1 animated wallpaper
- Guitar tuner
- Voice based language translator, handy for my international customers
- smooth HD video on youtube over VzW 3G
- smooth and fluid GOOGLE STREET VIEW with turn-by-turn navigation, equally good as a hand held GPS
and more and more every day
I can't wait for Google Earth.