I have the 26" Zillas and I have nothing but good things to say about them. The only complaint comes form the people who ride behind me on the trail. The Zillas will throw rocks and roost behind my ATV for days! Most people who ride behind me only ride close for a little while...
The traction is very good on all surfaces. I do mostly trail riding and river bottoms. I usually try to stay out of the deepest mud so I can't say how they are in that stuff but I'm sure they're pretty good. I don't feel like I've lost any power. The good thing about the Zillas is they are lighter than most tires. Even though they aren't radials, they handle very well on the hard pack and black top. I've had my ATV up around 70 mph on the dirt roads and it handled very well. Didn't notice any swerving or vibration either. I wouldn't ever recommend going that fast, I was only there for a moment as I was checking out the top end of my ATV.
As far as the difference between the 26" and 27". I don't know why everyone says they run small. I think they're right on size if not a little bigger. Here's a pick of them on my ATV next to stock ones. I'm not sure what the weight difference is but I'm sure it's not very much. You could probably get away with 27" without any noticeable difference.