I'm assuming that if you're "in window seats in an exit row" then you're traveling Coach. If that's the case, then it's really you're call, since the seats and service aren't too different between both the 767 and 777's. Unless it's changed, the JFK flight is via 767. The 2215 MIA flt is a 767 and the 2325 MIA flt is a 777. I actually prefer the 767 because fewer passengers make arrival easier. The only benefit I can see to the 777 is the in-seat video.