Hi tomfifield
I would suggest getting the most you can afford... I bought a JD MX-5 to do fairly heavy brush clearing, and have been pleased with it...
If I had it to do over, I think I would try to swing a Woods BB600 (although my PTO is rated for MX-5 maximum), the price difference was minimal... but everything is green!
Those 2 units are built like tanks, have standard stumpjumpers & slip clutches, gearboxes you could never overpower (with the 3400), have stand-up companies backing them, and can shred anything you can get your tractor over, although they are pricey!
Be sure you get one with a slip clutch instead of shear pins; check the thickness of the deck & sides--at least 1/8 inch on top & 1/4 on sides, easily replaceable skids are nice, get a 2" rating for cutting if you can afford (then you won't have to ask "should I have...") and you can choose bigger projects down the road if you wish, or at the very least, have a field mower that is sure to stand up to any accidentals you might run into.
My 2 cents anyway... best ones I found when in your boat...
Good luck (& have fun!) /forums/images/graemlins/cool.gif