[EVILWS6]

Welcome to the site fellow Georgian . Don't know much about the Trailmaster buggy. But the Kandi with the 110cc 3speed is not a GY6 engine.
My son has a 110cc Kandi, it's pretty good for him(7). The 1st gear is really low, 2nd is the one you'll use the most on trails once you get going, and 3rd is only useful on the very top of 2nd. Think of 3rd as a OD gear. It's a great little kids buggy for light trails, not sure how good it would be on steep hills though.
If you plan on riding in it with your boys on anything other than the yard or street, I'd buy a buggy with the 150cc GY6. Replacement parts are plentiful and cheap too for this engine. You always upgrade later to some performance parts as well to squeeze more out of it. I like the 150cc gy6 over the 5.5hp because of the options it gives you to grow.
As far as choosing between the TM and the Kandi? Not sure, I've never owned a TM. The only Kandi experience is with my son's buggy and when I called for a part they answered immediately and had the part to me in 3 days.
I do have a Hammerhead 250 SS, which is pretty much the same as the TM,but with a different engine. I do like it, and it seems to be built better compared to some other chinese buggies I've been around and driven. But haven't been around a 150cc or above Kandi for me to give a direct comparison against each other.
If you decide to order a buggy and put it together yourself, use lots of loc-tite on the bolts and nuts. That's the one thing I tell everyone who puts one of these together. I also change out all my rollbar bolts to 10.9 or 12.9 bolts, or any other bolt that breaks I'll swap out with these too.
Good luck on your choice and hope your boys love it. ( I know they will.)