Issue with 3G is that.. its so fast that MBs get gobbled up fast!!
No sooner u turn on 3G - few background apps consume 2-3 MB immediately in a flash!
Especially
iPhone is crazy phone when it comes to data consumption!
With 2G - you can check ur e-mail / whatsapp (basically text only stuffs) and those background apps wud not consume more than 500-700kb.