With the modular you have MUCH less to have to route behind the motherboard tray, I have one of each and I choose the modular EVERY time. And believe it or not you still want some air flow behind the tray to help cool the motherboard. Less traffic, less hassle, extra $$$ Always the way it works lol
Go modular if you don't have to skimp on other parts!
If you don't mind the extra cost, go with the modular or semi-modular. You can leave out the cables you don't need and make for less clutter and better air flow. And with most low cost cases, there is not enough room behind the MB tray to hide thick heavy sheathed cables. The finished build will look better too.