Iphone advice needed

[wantdaphone]

I have a 3G iphone that Is jailbroken and unlocked. I was using the T-mobile Sidekick plan mainly because I use the Iphone for data more than a phone. The sidekick plan no longer works, I've called T-mobile and that's what they told me. The advice I'm looking for is this: what other plans T-mobile plans work for Iphones? And would it be better to just get a 3GS with an AT&T plan? I'd like a something less expensive in a plan, but would go with AT&T if I could guarantee that I'd be able to use the 3G for the internet from my work. What I'm looking to do is use my Iphone to stream my music from my computer with something like Symplify. What would be a better plan to use and how fast would I build up to the monthly cap on each plan? Any help from others here would be appreciated, thanks.