You can view your usage on ATTs website to gauge for yourself what your actual monthly data usage is.
DH and I are with ATT and each have 2GB data and neither of us even gets close to using it all. We are on wifi at home and at work and 3G everywhere else. We don't stream music, movies, tv on our phones which is what uses up the most data. If you just surf and email then you should be fine with 16GB. I've attended 2 5 day conferences this year where I was on 3G constantly since the convention areas didn't have open wifi. Even in those 2 months the most I used was 280 MB for the month and that was with being on BBB, email, FB, etc all day long. DH hasn't gone over 70 MB in the past 6 months and he's on 3G at work because he can't access work wifi via his personal phone.
Mom to two amazing DDs ('07 & '09) and a fur baby.
Gluten free since Nov '11 after non-celiac gluten sensitive diagnosis. Have had great improvement or total elimination of: migraines, bloating/distention, heartburn, cystic acne, canker sores, bleeding gums, eczema on elbows, dry skin and scalp, muscle cramps, PMS, hair loss, heart palpitations, fatigue. I'm amazed.