Lots of variables.
How much can you afford?
How many acres do you need to brush hog?
How much time do you want to spend brush hogging?
Do you have any size limitations, fences/gates, low trees, etc?
How much can you afford?
Simple answer is that you should get the biggest tractor that fits within your constraints, where constraints are budget, any parking limitations, any limitations on trails, any maximum size you want for other reasons.
All good points,,now if you add some information like just what you want to do how much you want to brush hog the size of your brush hog,,, and most of all what can you afford to spend on tractor and equipment