IMO, get the .54cal.
The .45cal is my favorite caliber...don't know why but it just is...have more .45cals than any other and when I get ready to go for a weekend range session if there's nothing pressing me to do something with some other particular caliber, I reach for a .45.
Having said that, IMO, the .45cal is a small bore caliber if the only choices are small or large bore. I think of the .40/.45/.50cals as small bore (with the .50's foot on the line) and the .54/.58/.62cals as large bores.
In terms of the amount of energy delivered to a target...at distance...using a patched round ball, I agree with Mike that size and weight matters...big bores, big energy, big holes, and pass throughs.
I've shot deer with a .440/128grn Hornady ball using near max, very stout powder charges...all have been 60 yards or less yet I've never had a pass through...50's almost always pass through...but .54/.58/.62cals ALWAYS pass through...and IMO, pass throughs are best.
As a result, I feel a little undergunned going deer hunting in general with a .45cal and hope I'd never intentionally take a 100yd shot with it at a deer.
(yes, I'm sure deer have been killed at that distance with a .45cal, just like some deer have been killed with a .22 rifle)
So I only take the .45cal when I know I'm sitting over a trail where the max shot distance is 60-70yds so I wouldn't be tempted by a B&C trophy at 110yds or something.
In summary, as much as I like the .45cal, if you already have a .50cal, getting a .45 just means you'll have two small bore rifles (IMO) and for that reason alone, for a second caliber I'd get a .54cal rather than a smaller caliber. (Then you can add the .45 later)
The .54 is a solid performer to walk out of the house with, and have good energy at distance, plus it's similar in cost to shoot like the .50cal is.
A .58cal is to die for and will take most things on the continent at distance but a little pricey to shoot year round unless you cast your own round balls. (you can add the .58 later, after the .54 and the .45, etc)
My .02cents