On the 2G stuff: on GSM, 2G means EDGE. Which is tolerable for e-mail and other light data use. In fact, I think I was able to stream Pandora using EDGE on T-Mobile. But like others have said, I've read Straight Talk is known to be nebulous on what the data cap really is, so if you're a heavy data user, you may not be happy. But it's pre-paid so no harm in trying.
On Straight Talk in general: Buying the 1520 from AT&T and taking it to Straight Talk on AT&T is probably the only thing I would bother doing. Straight Talk on T-Mobile isn't quite as good of a deal. I think any pre-paid (including MVNOs) don't get the same roaming privileges as native T-Mobile customers. And T-Mobile has better native deals than all the other carriers anyway, so if I wanted to use the T-Mobile network, I'd probably just go straigh through them. Even if you want to do pre-paid, they have an amazing $30/mo plan with 100 mins/unlimited text and 5GB data. Plus their HSPA network is quite fast, which means you can be more patient waiting for them to roll out LTE in more markets.