3G is slower at 350-800kbps on average compared to WiFi (even the older 802.11b is 10 megs theoretical) . T-Mobile has no 3G whatsoever, so they won't offer any speed improvements. Verizon's EVDO (even rev. A) is about the same speed as AT&T's 3G. They're not really ideal services for streaming non-mobile video, though I find youtube works OK if I put the SIM in a notebook (using MEdia Net, not ISP. CINGULAR). But I'm in a strong coverage area.
Verizon Wireless' terms of service actually prohibit streaming media (other than any services they themselves offer-- i.e. VCast). Watching streaming video can get your account canceled, esp. if you use a lot of bandwidth/month