I'd suggest you to stick with TMobile, forget AT&T as it's too much trouble with its voice and data connection.
The iPhone will, sooner or later, be available for TMobile too.
And before going for the Nexus One, google some information about it. It appears the Nexus One is still affected by...
If you don't download the images in the emails, you'll be more than save with the 20MB x month.
I always go safe and get the 50MB so that in case I really need to download images or applications using the data connection there is always enough for it.