I've got both, a 54 that was store bought and a 50 that I built from a kit. They're the same size, 15/16" barrel and 32". The kit is basically as easy as the TC. The lock and trigger group are fully assembled and you won't have to touch them unless you want to refinish. The only real metal work you have to do is polishing the other parts including the barrel for finishing.
With the outside dimensions the same, one thing you'll notice between the two calibers is that the 50 is heavier, and the assembled rifle feels more muzzleheavy in 50 caliber. Since I liked the balance of the 54 better, I went to whittling. Fortunately, there's extra wood on the kit stock and you can thin and trim as you want. I was able to rebalance the 50 to match my 54, and along the way produce a more trim, eye-appealing stock.
Probably the only real caution is the tiny screws holding the escuchions. They're pretty frail. Lotsa guys replace them with the next size larger, rather than take a chance on snapping them off. I cleaned up the holes in the wood, then soaped mine and eased them in, backing off a turn any time I ran into resistance. They went in fine.
There's a mountain of past threads about building and finishing the GPR kit here on the site. If for no other reason than having so much advice and so many examples of finishes to draw upon in the archives, the GPR kit may be the best of all for you to build till you get a little more experience.
