I have been a loyal customer of T-Mobile for many years because I get the best bang for my buck and great customer service. However, they have recently been locking down tethering abilities; TM506 firmware updates have removed this ability, and they recently asked Google to remove tethering apps from the market. So, I would be careful or think ahead before making a decision. You'll definitely need the technical know-how to tether with the G1.

In regards to having a physical keyboard: again, I recommend the G1. The thumb keyboard for the N800 drives me nuts, but the G1 physical keyboard is awesome.

To compare, here are my initial comparisons between my Nokia 6680 and the HTC G1 (what one does, the other doesn't):

Nokia: Tethering out of the box
Nokia: Shoots video
Nokia: Faster camera with flash
Nokia: Ability to shoot video
Nokia: Battery life better
Nokia: Can send files (like photos) via bluetooth (G1 can't!)

G1: Better support for maps
G1: Does 3G on T-Mobile
G1: Slower camera, better quality
G1: Keyboard
G1: Industry-standard plugs (USB for power and connection)
G1: OS hackable