Just wondering how you decide between the 2. Is there any advantages in using a 45 caliber ML, or a 50 caliber ML?

More options for the 50. By far.

45 is good if you want to shoot 45 conicals or are definitely going to shoot the .357 or .40 caliber offerings. A 45 is usually heavier (outside barrel diameter the same) and I'm not sure there is any real ballistic advantage to it though I believe you can get a bit more velocity.

I have two .45 and five .50 . I can get 150- 200fps more with a .45 than my .50 using a 200 grain bullet with the same amount of BH209 – I get around 1800 fps with 83 gr of BH209 in my .45. The .45 has less recoil due to less powder. I think it is because the smaller barrel builds more pressure. The .50 has lot more bullet choices to pick from.

IMO if you are hunting thin skinned animals like deer, and your shots are extreme distance, and we're talking.. 150+ yards, there might be some advantage to the .45 caliber. They claim with the modern sabot round they are flatter shooting.

But here's my point. If your shooting the same weight bullet as the .45, it might have an advantage. But lets say you are shooting a heavier bullet because of the larger caliber and selection. You have a scope on your .50 because you expect longer distances, you have practiced, and you can hit your targets even with more of a rainbow trajectory. At impact you can push a larger bullet that carries more energy.. So where is the advantage?

The one place I see a .45 maybe topping the larger calibers is in a White Conical shooting rifle. Both my .451 & .504 shoot a 460 grain bullet. The .451 being longer, and shorter diameter, and more aerodynamic. Both can be pushed to the same powder charges. Here, the .45 might have a real advantage over the larger caliber. It should be flatter shooting, but hit as hard. And the difference in the size of the hole.. well who will notice when the deer is on the ground.

No there is little advantage to the .45, bullet selection and sabots is greatly reduced, the price is about the same so why do a 45? A 50 will do everything that a 45 will do and then some.
Chap

I wouldnt mind using a 45. Id like to play around with the .357- 358 bullets. You get a 200 grain bullet going 1800 to 2000 fps. It would be like shooting a 35 rem . with the .358 bullets theres a good selection of bullets. Other than that No reason to get rid of the .50. I wouldnt mind picking up one cheap just to try out.
Now heres something to think about. A .45 in a small package, 20 inch barrel ,shooting a 200 grain .358 bullet.
Might make for a nice little canoe gun . or treestand gun . Just thunking outloud.

I know exactly what you mean, I went to a 21" Green Mountain Barrel on my TC Flintlock, and it is very light and easy to handle. Accurate also in the range I want to shoot it.
Chap

There's not much of an advantage to the .45 unless you want to shoot .357 - .358 caliber bullets- the .45 can do that, the .50 can't (unless you use a duplex sabot). However, I really do like my .45 and plan on keeping it a long time. My next purchase will be a .50 though, - much better selection of bullets.