For me, 3PH vs pull type has little to do with backing.
If I could guarantee that the only thing I would ever use it for was open pasture with no foreign debris, then it would be fine. But using mine for business, I like to be able to raise it all the way up in a moments notice if I hit something. Stump, fencing, chunk of concrete, bicycle, etc. And also having the ability to to raise it, back over, and lower it onto thick brush/briar patches.
So the choice of 3PH vs Pull should be made AFTER you analyze what you will be cutting.
Now as to cutter size and HP. First and foremost, it isnt an exact science. All cutting conditions are different. And no matter how much HP you have, or how small of a cutter, you will always find some conditions that will bog it down. As evident that some on here state a 7' is a load with 70hp. (10HP per ft). Even if you double that and go 20hp per ft, there are going to be times it is a load.
I run a heavy 6' cutter with 29 PTO HP. And while there are many times I get into thick stuff that will slow me down to 3rd gear (2 MPH), FAR more often I am slowed down by terrain being just too bumpy. I like mowing in 6th gear, which is 5.3 MPH. And frequently have to slow to 5th (4.1 MPH) just because it is too bumpy, or am maneuvering around trees, buildings, etc.
My point is simple. It is my opinion to get the biggest cutter you can run in lighter conditions. Cause you can always slow down. IF you size the cutter for heavy conditions, there is a limit to how fast you can speed up when conditions are light. Those guys running 7 and 8' cutters with 70-80HP, thats fine if you are always cutting heavy stuff. But what about something like this? I was able to mow 6th gear, 5.3 MPH, with 29HP and 6' and it wasnt a load at all. I have no doubt that I could have pulled an 8' cutter just as fast.
<img src="http://www.tractorbynet.com/forums/files/buying-pricing-comparisons/387954-tractor-pto-7-foot-cutter-img_20140815_091918_729-jpg"/>
With 80HP and an 8' cutter, In theory you could go faster. BUT, terrain wouldnt permit much more.
These are just my opinions and observations based on mowing 400+/- acres over the last few years in every type of condition imaginable. And while some may thinl 6' is too much for my 29HP, I have cut some really nasty thick stuff and still have never had to touch 1st and 2nd gear.
You would also be surprised at just how much less power is required by mowing just a 1/2" or 1" higher.
I am actually considering upsizing my mowing outfit as well. And will likely be an 8' twin (for towing purposes), and the L4600 or MX5100 is about the size I would go with.
