After reading online it seems the "Postpaid" plan can be canceled at any time but you need to pay every month rather than use it only as needed. No contract, but no "pay only when needed" and no $15 250MB option.
What I'm curious about, and I'll probably explore it sometime, is this: If I add the ipad to my family plan as another line (AT&T) does the data plan get the 20% FAN service discount my other data plans get? Can I manage features online and turn off or change the dataplan and get prorated for the month, turning it back on sometime the next month? Lastly, what sort of surcharges and taxes am I going to incur on the line for the ipad that I'd avoid in prepaid?
I suspect I won't find out these things unless I just go ahead and get it set up as postpaid and try it out. Be interesting to see. Lots of stuff to weigh back and forth between the two options.