Unfortunately T-Mobile had to use AWS for 3G, there was no way they could have skipped 3G and gone straight to LTE, they would have no customers rights now.
Luckily they will use AWS for LTE though, so in that perspective the 900 will work on T-Mobile LTE when launched in 2013. And they are refarming 3G to 1900. But you probably won't see that till the end of the year.
As for pricing, I unfortunately (or fortunately) get corporate discount thru T-Mobile, so even if I could manage to get corporate discount on AT&T it just won't be cost effective.