IMHO go for the unlimited at first. See how much you use and then at the end of the thirty days see if the 250mb package will do. In less than 24hrs I have already used 92mb from AT&T and i've been home using wi-fi 80% of the time. At this rate I will probably use 250mb in less than three days. The way the plan is set up, if you don't have unlimited, it will get really expensive. At the end of the 250mb you either have to buy another 250mb for $15 or spend $30 on unlimited. So now you are basically spending $45+ for unlimited. Lucky for me I took the advice of another poster on this forum and went for the unlimited right off. iPad support 3G support group comes through again!
I've used 58GB in 24 hours by sticking to wifi and only turning 3G on when I have to. Why is this so much higher than my 2G iPhone? Faster and so many more MB for the HD resolution and size? My guess. Not sure if I'll upgrade to unlimited or just let it die until next month. When I run out, I run out; suck it up.