A 6 foot machine would be the best choice in my mind, as it also makes it easier to get into tighter spots. If you have a large area to till, the difference between the 6 and seven foot may end up with you having to make up perhaps one or two extra "passes" to compensate for the difference in size, as well the tractor would probably utilize that size better if you were to get into some tougher conditions. We used to run a six foot Howard roto-vator behind a 4020 JD. and there were times breaking prairie that it would make that tractor snort..... but then again, also ran that same tiller behind a David Brown 995 ( 60 or so HP) and it worked well especially for doing local gardens for the neighbours but being six foot I could get into some tight places. Just my opinion.
Cheers
Roger