Go with a 50 cal or the 45 cal. If the 45 cal was the same outside diameter barrel as the 50 cal, then go with the 50 cal. Most commercial gun makers use the same stock for multiple calibers. you're just adding more front heavy weight with the thicker barrel.
Also, if you go to those muzzleloading competition, you might be more challenged with a 36. The smaller ball can cause more very accurate misses, like tie breaker paper targets, hitting small things like loli-pops, or splitting poker cards. Also, some places use knock down targets to score. If your small bore doesn't knock a plate down, but hits it, it's still a miss. Places with a lot of shooters can mistaken a hit for a miss, because they didn't see the larger farther away targets swing and they weren't sure if the ring they heard was yours or somebody else's.