You'll probably get a range of recommendations on this one. The variation has to do with how much time various people are willing to spend mowing 25 acres, and how much money they're willing to spend to minimize that time.
If you are happy with the time you are spending with your current set-up, you could look at another 4 foot brush hog set-up. Some of the sub-compacts (New Holland, Kubota and John Deere and others) will handle this.
My bet is that with that much to mow, you'll want to step up in HP a bit. If you want to run a 5 foot brush hog, you'll probably want in the 25 to 30+ HP range (you can get by with less if you mow regularly, so the grass doesn't get out of hand, or are willing to go slow &/or overlap your cuts a bit). I'm most familiar with the New Holland brand... Look at the TC29 or TC33 (or the TC30 if you are looking for more of an economy tractor). I've got a TC33D which handles a 5 footer with no problems. In the JD line, Check out the 4310 (or the 790 for an economy tractor). Kubota makes some great tractors in this size range as well, though someone else will need to chime in with model numbers. Those are the "Big Three" brands. There are others you may want to look at as well.
For a 6 foot brush hog, consider something in the 40 HP or higher range (again, you might get by with less if you mow regularly). A NH TC40 (or a used Ford/New Holland 2110/2120) would do the job. In JD, look at the 4500/4510 or 4600/4610 (or the JD 990 for an economy tractor)
If you are thinking much bigger than that, you're getting out of my range.
John Mc