Better Internet? T-Mobile $30 or Sprint $15

I already have had Sprint for several years, but I really want the T-Mobile DASH phone. Sprint has a good basic internet package for $15/mo, nationwide. The T-Mobile internet package is $30/mo. I don't need the HotSpots at all, in fact, I NEVER go to Starbucks. I just want to stream Shoutcast radio on the Dash.

I have looked at the Sprint Treo 700p, which is fine except no wifi. Dash is great but $30/mo for basic internet access.

T-Mobile Internet $360/yr
Sprint Internet $180/yr

So, which is the better internet package, Sprint for $15 or T-Mobile for $30? Is the T-Mobile coverage twice as good as Sprint, since the price is twice as much?

Sell me on T-mobile coverage as opposed to Sprint! I need to decide real soon.

Thanks,
D*