Since you need the scope to compensate for vision, I don't think either gun (as they are currently configured) would be the best for you.
As far as .54 caliber goes, "I think" it's one of the most efficient bore sizes above the .45 Both round ball or conical (saboted bullets too) are quite effective on deer and larger game. I don't think anyone would argue that point. But if availability ever became an issue, a .50 caliber is the equivalent of the .30-30 in cartridge guns. Not ballistically of course... in that I mean that if a store only carries one caliber of round ball or conical (or sabot), it's most likely going to be the .50 cal.
You give up some power by dropping down to a .50 caliber, but you gain a LOT of versatility and variety of projectiles, accessories, et cetera...
Most sidelock guns aren't conducive to scope mounting without some modifications. There are problems to consider with hammer interference and high mounting. It may also put you into an uncomfortable position when trying to shoot.
There are some replacment hammers available for certain model guns that have an offset spur to allow for scope clearance.
It might be possible that you'd be best served by a different type of muzzle loading design gun.
Example: An H&A Underhammer .50 caliber gun would work great. Though most of them are 1:66 twist, you can get them fitted with just about any barrel, caliber, twist, combination you could possibly want. Scoping it would be very easy and you don't have to worry about hammer interference or mounting issues. They also are handy for left-handed shooters.
There are a whole slew of "modern designed" inlines in various brands and prices. Most have much faster twists needed for shooting saboted rounds. (long conicals might work well also) They aren't the best for patched round ball shooting, but that doesn't necessarily mean you can't.
I'm not trying to change your mind about what you want, but just giving you some alternatives & ideas to think about.
Shoot Safely!
WV_Hillbilly