If I had Sprint, I'd get an Evo for sure. I have (and love) a Nexus One on T-Mobile. There are things I think I like more about the Nexus One (judging by pictures and reviews), but the mobile hotspot feature plus the possibility of 4g looks compelling.
However - to answer the original question - yes, I'll most likely get the 3g version. The reason is that the iPad has a 10+ hour battery life. If I remembered to charge my cellphone before I leave the house, it probably has two or three hours of life while actively handling data, and it might be less than that while both exchanging data with cell towers, and exchanging data via wifi with a device like the iPad. The whole point of an iPad is simplicity - with strong battery life a huge selling point in my mind.
I'll be interested to know how 3g affects battery life - I'm sure it'll cut into it, but I'm also sure it will still easily have more battery life than a smartphone.
4g would be nice, but I can wait a while ... I am not all that enamored with Sprint's approach, which I think is rushing an inferior technology to the market in order to grab attention ... Sprint's 4g is wimax that has a max speed of 10Mbps. The other carriers are mostly working toward deploying LTE, which has a max speed of at least 100Mbps. But then, Sprint's service is available today, and the others are not.