Well to answer in short...you will have problem playing MMORPG.
I'm on verizon and I'll tell you one thing... Their 1xrtt is not a real 3G network. It's more of a 2.5G and although you will get faster than dialup speed... You're really getting it in burst shot instead of constant...This mean latency is a BIG problem.
I don't know about sprint...But I do love their data plan.
And I don't think GSM have a good 3G network yet.
If you live in the Washington DC area... I think Verizon is testing their EVDO network... Which is REAL 3G network... I think that would be fine for online games... But it's still not widespread yet...
If you're planning to just browse the web @ faster than dialup speed...Then the cellphone route is alright. But remember, Verizon uses your minutes and Sprint don't. That means with Verizon, you'll only get unlimited usage @ nites and weekends.