One offers 250MB per month for $14.99, and the other offers 2GB (a cap eight times higher) for $25. Both will thankfully be available on a month-by-month basis, so you won't be forced into a contract of any sort. This all comes on top of the $50 premium that Sony plans to charge for the 3G model; the Wi-Fi-only model will cost $249.99 as compared with the $299.99 price tag for the 3G system.
AT&T didn't provide information regarding overage costs, nor did it mention any caps on downloads. In Japan, only files up to 20MB in size can be downloaded over 3G; anything larger has to be downloaded over Wi-Fi. AT&T has a similar restriction for iPhone users, who were initially capped at 10MB until that was raised to 20MB just prior to the launch of the iPad. AT&T will be offering its Vita customers free Wi-Fi access at its hotspots around the country, of which there are more than 29,000, which should alleviate some of the pressure to download content over 3G.