If I was buying again, I would probably go with the 54" like Will C has. The 60" Bush Hog I have is fine as a mower, but it is a lot of weight and it is way back, so I would be willing to give up that much cutting width. Going to a 48" is exactly the wheel width and I would not opt for that unless I was mostly cutting trails or cutting really heavy stuff. You will find much less choice though with the 54" as most are in full foot increments. The only choice I know of is Woods, which is a good one, but may be more expensive than you want to pay.
In case you are not aware, using a rotary cutter with the 1/4" valve on the B model requires that you keep an eye on the front of the cutter and bump it back up every 10 minutes or so. The rotary cutters only have one wheel in the back, so the front of mower is held up with the 3 pt hitch. The 3 pt hitch with the 1/4" valve will not maintain a constant height over a prolonged period, so if you don't bump it back up, you will look back and see you are dragging the front of the mower in the dirt. I have chains rigged to help hold mine up and when they get tight, I still bump it up to keep all the weight off of them.
As mentioned by Tripple R, usually someone weighs in on the advatages of flail mowers. I know nothing about them, but they do ride on the ground on skids on the side and a roller in the back, so that should eliminate the issue with the 1/4" valve. Again, I know nothing about them, but if you are mostly mowing grass in fields, they may be worth a strong consideration. If you are also wanting to go through heavier brush, the rotary mower may be the better tool.