I agree with Billy_S. Buy what you can afford and you really don't have any issues. I purchased a 6' KK Rotary Mower for use on pastures with grass, small trees (just sprouting) and blackberry brambles. After 19 hours of use, I have no complaints. The XB stuff is a little on the mild side for use in "rough" environments.
You won't want to cut 2" saplings with any cutter on a regular basis. That is hard on the drive line. Both the gear box on the mower and the PTO on the tractor. The caveat here is, if you use a slip clutch, you won't have nearly as much trouble with the PTO side. I've sheared 3 grade 2 bolts used for the drive shaft on my KK. It's about a 10 minute procedure to take apart the shield, remove the old bolt piece(s), install a new bolt and assemble it all again.
If I had it to do over, it would be about twice as much money for a JD MX6 or somehting similar. I would have a tough time using a JD for cutting in rough terrain paying that kind of price. I'm comfortable with my KK purchase. Good value for the price paid ($810 for 6').