I think you'd be hard pushed to get a 60mm 240 rad in the top of the 540 AIR. I've got a 47mm 240 (EK CoolStream XT) up top and that pretty much comes down to the top of the motherboard. The next 13mm or so would probably depend on the motherboard. In my case (Z87X-OC) I think I'd have problems with the 8-pin power connector.
There's probably not a lot in it between the 2 cases. I think for me though, I'd go with the 750D for watercooling (I should've waited). The 5½" bays are in the right orientation for a bay res (if you wanted one) and you don't have to deal with either trying to fit the pump in the windowed side or routing tubing between the 2 sides if you put the pump in the PSU side (like I did).
I'm just about talking myself into switch cases (dammit!).
For air cooling I'd go with the 540 AIR though (duh!), so it might be worth considering if you're ever likely to use the case to air cool a build.