Personally if it were just one tank I would go with 2 X 36" -- gives you a bit more flexibility, can angle them differently if u wanted to, or you could just turn on one of them for even less light before bedtime or when medicating. Also if one goes out, cheaper to replace than a bigger one.
But with your gorgeous double decker there and 2 tanks, in fact I'd probably lean toward 1 X 72" for each tank; it'll be neater and less wiring to worry about is my thinking. Look fwd to seeing more photos!