Are you asking which one to buy? Bigger is going to be better. More weight and you get more done faster. With 45 hp, you could go bigger. I have a 35 hp tractor and my 5 ft disk is way smaller then what I want. I got a fantastic deal on it, so that sort of forced my decision to buy it, but I'd much rather have a 6 ft disk, and I think I could handl a 7 ft, but don't know that one for sure. With 45 hp, I'd be looking at 7 or 8 ft.
Another consideration is how big of a food plot will you plant? I have two that total just over an acre. I plant my food plots twice a year, spring and fall. I disk, fertilize and disk again in a few hours time with my five foot disk. While bigger would be nicer, it's not really worth paying more money for one when I use it so little.
Eddie