Data won't always be faster on 3G. It depends on how busy the cell is. Sometimes you will find it better to fix it on 2G.
Battery wise, 3G will be worse, but you should find the newer phones better, and you should still get several days standby. It also depends on how strong the signal is where you live, weak cells use more power from the mobile. But it's the display backlight which eats most of the the battery if you are browsing and using maps a lot.
re 500Mb, that's a lot of data for a mobile. Unless you are downloading music and videos stuff you won't have a problem. If all you do is stuff like maps, assisted GPS, and static web pages, you are unlikely to have a problem. There is a data counter on the "logs" page of the phone so check that out if you have doubts. I browse about in idle moments, Google things, use Google maps and sportstracker sometimes, but never any videos, and only a few photos to Flickr. That's stacked up 150Mb in two months.
BTW1 you might find the 3G system locks you out if you spend time high up (eg riding up on ridge tops etc). If your phone can see too many 3G cells, the system gets in a mess. I find I sometimes need to re-set the phone (or run 2G) if I'm in placers like that.
BTW2, Asda PAYG does 20p/Mb data rate for anyone who wants to keep off the contracts. - I just got one out for Mrs MC.