I think a lot of manufacturers (tractor and implement) are too conservative with their ratings.
I used a 5' Land Pride behind a 16.5 PTO HP Deere 670 going through brush that was thick and tall (as high as the cowl). I went very slow (1st or 2nd gear low range). It did a fine job too.
If one uses common sense when operating, they should have very few problems with equipment that is reasonably sized for their tracto.
So, go with the 5' cutter...