The PlayStation Vita comes in two varieties: one equipped with wi-fi, and another pricier model that includes mobile 3G service. While AT&T has yet to detail their plans for the US release of Sony's upcoming handheld, this bit of info may sway your decision. 3G downloads will be capped at 20 MB, so larger games and files will have to be downloaded through wi-fi.
A Sony representative confirmed the size limitation to ASCII (via Andriasang), but said that the number could change sometime down the line. 20MB would only let you download Minis. Current generation PSP games can be as large as 1.5GB, and it's likely Vita games will be even larger.
Sony also addressed concerns that 3G speeds wouldn't be conducive to online play for some games, like fighting games, due to latency. The rep said Sony is providing developer support to ensure against technical limitations, but didn't give specific details on its plans.
Given the restrictions placed on 3G downloads, it will be interesting to see how the 3G sells compared to the cheaper wi-fi version. The Vita 3G will allow you to check in with social networking apps like Twitter and foursquare, but will consumers shell out an extra fifty bucks for such limited download capabilities? Judging by the less-than-enthusiastic reaction to the announcement that Sony partnered with AT&T for the 3G, it doesn't seem likely.