Hi can some one answer my question about the psp vita ngp before i pre order it?

hi should i get the psp vita with wifi or 3g with wifi. what is 3g exactly.would i really be able to go on the internet where ever i go. and since i have internet with AT&T will i be able to use the psp vita internet where ever i go without paying just to use the psp vita internet while paying for my home internet.Please answer you can also give me the pros and cons about the psp vita