Scottie,
Bird has raised a good point and I think you should give some further thought to whether you might not be better off with a 5' mower. I wouldnt get the 6' solely because that's what your dealer has available. He shouldnt sell you one solely for that reason either. (Not saying he is.)
I'd say there are 5 things to consider:
1. Can your tractor lift the 6' mower when you are transporting it. That will depend on the weight of the mower (get the exact weight) and the lift capacity of your 3ph.
2. Even if you can lift it, can you comfortably and safely drive it with no fel or weight on the front of your tractor. Bird mows without fel or weight, but he has a larger, heavier tractor and a smaller, lighter mower than you are considering. If you end up rear heavy two things can happen. You lose good steering ability. More dangerously, you have the chance of tipping over backwards when you go up a hill. This then partially depends on the character of your land.
3. Can your tractor pull the mower. I'm sure it can on flat ground, but you have to consider the contours of your own land. If you have hills, the tractor will have a more difficult time with larger implements.
4. Scalping. The wider mower will have more of a tendency to scalp the lawn if you have dips and gullies and small mounds. If you are very flat, that wont be a problem.
5. Maneuvering between and around things. The rear mower in general is more difficult to maneuver around things than a midmount. The wider the mower, the more careful you will have to be. Also, you may not be able to squeeze through some places with a 6' mower that you could with a 5'.
Only you can judge these factors. My gut instrinct without knowing your land is that the 5' is more properly sized for a 7500.