Both sizes will work, the difference being the 6' is heavier and I am not sure if that will make the front end light on your tractor as configured. Keep in mind the weight of the mower, relative to the front axle, a tractor may handle a larger or heavier duty model, but you may need front weight or get one that is better suited to the tractor you plan to use it on. Performance, you will have more power available to the 5' size and it will likely shred better, but that depends on what you are cutting. I've used a 6' model on a similar tractor for 13 years or so, and even going up hill, tall thick hay like grasses, will be all the tractor wants to deal with, but it will cut it quite well, at a reasonable height, lower you go, the more power you will need, same with the width you are cutting, take less of a bite and you will notice the difference. If you are maintaining pasture or areas that are not left to grow wild requiring clearing, the 6' size and that tractor will work fine, its the heavy thick grasses/weeds or vegetation that will bog you down, and that is where the 5' model will likely do a better job. The drawback is the tractor width, what you run over will won't cut as cleanly.
There are many manufacturers of these, I like Rhino myself, (see link below), but Bush Hog, Woods, Howse, King Kutter and how many others out there offer a good medium duty, mounted, (3pt) rotary cutter, you'll have to compare, check pricing and dealer support.
Bush Hog and Woods have a good reputation, we sold the previous a long time ago, both Bush Hog and Servis-Rhino has been building these a long time, so has Woods.
I have a Rhino dealer nearby, when I upgrade to a new one someday, it will likely be a Rhino. I've run an older Rhino SE-6 that has seen a some acres before I got it, they are durable and this one is not heavy duty. I like the new deck design, preferably all implements are to be kept under a roof, but if not, this one, if cleaned after useage, will shed water off the deck. Gear boxes are stout and they just seem to be a well built rotary mower. I'd still look at the others, its nice to compare and see whats what and be comfortable with what you buy no matter what manufacturer.
Keep in mind, for best performance, the mower needs to be properly adjusted to the tractor, and new or sharp blades maintained. Be cognizant of field conditions and obstacles. I don't like areas I don't know, so I'll walk them to look for hazards and or cut with the deck up high.
I would advise getting safety guards if they do not come standard with the mower, these things have the potential to launch things out, though the one I run has been great about that, the trajectory is evident, mine has no guards so I keep rpms down and cutting height up when in areas that I know will toss stalks or rocks. With recent discussion of guards on this site and the fact that I saw a softball size round rock come out the front of a bush hog brand cutter with no guards in May of '82 as I recall. It sent it up over a hedge row into an old pasture, they are necessary. That rock would have killed a person instantly, hit like a home run, out of the park! Guards will dampen the projectile or stop it, protecting both the operator and the tractor, especially the rear tires.
Rhino
Bush Hog
Woods
Howse
King Kutter