Does T-Mobile's network in the US allow an iPhone 3GS work as well as the iPhone 5? Buying an unlocked iPhone makes it cheaper to find a iPhone 3/3GS than a 4 than a 4S than a 5. Please assume that I know and am aware of all the other benefits of newer hardware like cameras, running iOS 6 (in the case of the iPhone 3), more RAM/faster CPU so that cheaper hardware will likely be both older and slower. However, functionally with the network connectivity, is there a difference?

But for someone that will only call, text, use maps and very lightly use Safari, my thinking is that I shouldn't spend any extra money and get the oldest phone for a friend that is constrained to T-Mobile.

Does the T-Mobile network / infrastructure only work with 3G or am I incorrect in thinking that newer phones don't work any better with T-Mobile's 2012/2013 network frequencies?
No He will be stuck on IOS 4 most popular apps have support for ios5 or higher
I have a 3g 16gb too old espically for a newbie. Get the 3GS unlocked version if you want to be cheap/ most bang for bucks. He wil be stuck on EDGE until Tmobile finishes adding 3g Support for unlocked iphones. They have done a few cities google which ones 
16/32gb 3GS is the best in my opinion. Favcetime and stuff is impossible over 3g unless you can find a ip4 off contract for sub 200 dollars then get tyhat i used  4 different iphones on tmobile 2g 3g 3gs ip4 been with them since 2001