It's cheaper, but not significantly cheaper. A basic plan on AT&T would cost you $70 for data + 450 minutes, T-Mo would run you $50 for data + 300 minutes.
There are ways to get data cheaper on both carriers, but these are the standard 'official' pricing. T-Mo has better text plans. $5 for 400, $10 for 1000, $15 for unlimited vs 200 for $5, 1500 for $15 and unlimited for $20 on AT&T.
I'd love to have the iPhone on my wifes SERO plan. $30/mo for unlimited data, text and 500 minutes. $27 after taxes with her discount. Even my old plan was $55 for 450 minutes, unlimited data and text.