You're actually looking at a number of variables beyond just the engine horsepower. Generally speaking going from 35 to 40 horsepower means you go up in frame size (again, just as a general rule). The larger frame will normally have a longer wheelbase, and often bigger wheels/tires, and both help make for a smoother ride. Also, the bigger machine will often be a bit taller, and that can help get you above some of the flying stuff that comes out from under any sort of mower.
If you can swing it, a cab will make a huge difference in how you feel after a day of mowing....clean air, controlled temps, no worry about hitting hornet/wasps nests, etc. I have no doubt that at the end of the day, you'd be more comfortable after running a 35hp machine with a cab than a larger 45hp machine open station.
As far as power goes, match the mower to the PTO power and it really shouldn't make any difference other than how long the job takes. Most 35hp machines should be able to run a 6' rotary cutter just fine...you need around 30hp at the PTO to run a 6' rotary cutter unless you're just cutting weeds/grass. Going up to a 45hp machine might give you enough power for a 7' cutter, but they're longer, heavier, and pretty unwieldy in comparison. In fact, an 8' cutter with two spindles is shorter, and takes less power to run than a 7' cutter with a single spindle.
My LS had 38.5hp at the PTO and my Massey has 45hp at the PTO and I can't tell any difference in how they run a 6' medium/heavy duty rotary cutter. I also ran a 5' rotary cutter behind an 8N Ford with 20hp at the PTO and it was okay, but would bog a little in heavy brush (didn't really use it to clear anything heavier than saplings).
