I know a few folks that have both Knight and T/C ML inlines in 45. None of them complain about having a 45 versus a 50.

That said, if I were buying new or new to me I would be looking more towards 50cal.
There is so much more development projectile-wise in 50 cal than any other ML caliber.

But that does depend on the deal also. I would not likely walk away from a really great deal on a quality 45. If fact, I missed one last year that I sill wish I had snagged up (T/C Encore SS in 45). I guy I know got one for Christmas last year from his wife and simply hated the sport that is muzzleloading. He sold the gun with a Leupold VX-III 2.5x8 for $500.
Then he caught hell from his wife and felt like an idiot after she found out because she had dropped almost a grand on the setup.