T-Mobile's web page says "Get 4G Speeds". It doesn't say that T-Mobile actually runs a 4G network, because they most certainly don't. What they have done is increased their 3G speeds -- in some cities -- so that their 3G network runs about as fast as a 4G network. But again, T-Mobile is NOT running a 4G network.
And EDGE is an option on T-Mobile. Matter of fact, the only in-building data coverage T-Mobile has where I work is EDGE. You walk outside of the building, and it switches to 3G. This is important for you, because an iPhone can only work on EDGE with T-Mobile. T-Mobile uses different frequencies for their 3G here in the US, and the iPhone does not support those different 3G frequencies. It does support T-Mobile's EDGE frequencies.
Neither T-Mobile nor AT&T sell specific "3G" or "EDGE" data plans. They simply sell "data plans", and you can access them via either 3G or EDGE, depending on your phone and their network. Regardless, it's the same price. You don't pay more for 3G, or pay less for EDGE.
If you don't notice the obvious speed difference between EDGE and 3G on T-Mobile, then your area must get horrible 3G speeds.