I'm sure you've already checked your own coverage on T-Mobile's coverage checker, but the main difference between prepaid and postpaid is roaming as it's already been mentioned. Some areas only postpaid T-Mobile phones will roam onto another carrier (like AT&T) to complete calls, but if you mostly stay in the city, that shouldn't be a problem at all.
Personally, I really would not rely on Skype over WiFi for calling if you are going to use significantly more minutes than just 100 on the $30 prepaid plan. It's not going to be as reliable as just using the cell network. As it's already been pointed out, extra minutes over the 100 are just 0.10/min which is not bad.
I would do the Value plan if you know you'll use those 1000 minutes and mobile2mobile calling. You'll also be able to talk to 'regular' customer care (prepaid is outsourced exclusively, on postpaid you can still talk to reps in the US if you need to) and you can add a corporate discount (I get 14% off) if you or your wife qualifies for it. Plus you'll be able to add unlimited/free WiFi calling via T-Mobile's own app (which works well BTW) so if you have WiFi at home, you'll have unlimited minutes.
How big a deal is the roaming issue? For instance, if I'm using my GPS on a road trip up 95, am I going to periodically lose service? The covergae map in my area looks good. Will I ever need to roam if the whole map in my area is good or better on T-Mobile's spot?