Work getting done now is factored into the 2 weeks missed hence the 156 vs 154 weeks. A fun factor could also be weighted in with a similar analysis, leading to the same ratio required.
My plan is to buy the iPad when Apple sends out invites to their Sept conference. They usually send em out ~1 week before the conference. That way if a new iPad is announced, I'll return the iPad 2. Otherwise I'll keep the iPad 2 and enjoy it.
I can't even physically buy one until mid-Aug (currently on internship on west coast, ~$200 gift cards are back east), so why not wait to see what the Sept conference holds?
Smart shopping can help save a lot. I'm sure your iPad 1 works just fine. But one functionality difference that quickly comes to mind is the front facing camera. Good way to chat with the parents back home

. I think that alone makes up the .02 difference necessary.