The right/best answer really depends on how to you plan to use the machine and what type of terrain and conditions you would expect to use it in.
For my intended uses, the bigger machines have far more checks in the "cons" column as compared to the "pros." As others have noted, the main advantages the bigger quads have over the smaller is load carrying and towing capacity. I would add clearance and weight, the latter of which is an advantage if crossing deep, fast rivers, but a massive disadvantage in many other situations and types of terrain.
I think a 450 (400-500) really hits the sweet spot for an ATV. Big enough to haul you and as much gear/meat as a personal could reasonably want to, but small enough that you can wiggle it into and out of just about anywhere and manage it by yourself. Throw a winch on the front and some ATP Mudlite tires on it and you have got, what I think is the ultimate access and tool, at least here in Alaska.
The smaller machines don't get stuck as easily, and if they do, they are much easier to get unstuck. They are a lot easier to manage on steep, loose, off-camber terrain. They get better fuel economy that their larger counterparts. They fit through places that ****** machines won't such as some of the pedestrian/railroad bridges. Trailering them is easier.
I have no need or interest to go fast. Not really possible in most of the places I wind up.
The main limitation I run into with the smaller machines is crossing deep, fast rivers. The bigger machines definitely handle those with a little more comfort with higher clearance and more weight to keep them in contact with the bottom. That having been said, if the crossing is deep enough that I can't do it with my machine, I usually wouldn't consider it even with a bigger machine.
I've spent a ton of time on Honda's (400-450 Ranchers and Foreman) and Yamahas (Grizzlys and Kodiaks 450 & 700) and would recommend either. No major issues with any of them. IRS is your friend in rough terrain. Heated grips are a must if using in all seasons.
"What is the best offroad vehicle for Alaska" is a debate held many times daily I am sure. For me it is the one that opens up as many types of terrain or as much access as possible rather than a vehicle choice that might be really capable, but only for a specific application or type of terrain.
I currently have a pair of Grizzly 450s and have had them loaded up with gear on the racks while towing a trailer stuffed full of moose meat many times and they had no problem handling that.
The little quads, in my opinion, will get you more places than any other option.
The 6-wheeled sit on tops like the Big Boss are quite popular and strike a pretty good balance between a lot of the different considerations. A 6-wheeled SxS with tracks is very versatile and capable as well, but $$$. Sherps are pretty BA but impractical for most.
