You can hardly go wrong with a Chambers kit. Jim is a true gentleman, and will bend over backwards to make sure you get quality wood, and parts.
With a musket, fit is everything. Tradeguns almost never have enough drop to keep them from hammering your cheek. Since most tradeguns, and fowlers, are quite light, the bigger gauges can be real face busters too.
I believe that if you shoot a lot of shot, rather than round balls, you may want a larger gauge. This depend a lot on what you're shooting at. Clays, and upland game, can all be taken nicely with a twenty gauge. Water fowl are bigger, and harder to kill cleanly, and usually require a bigger gun to do the job.
Hungry Horse