Personally, I'd definitely go drop in sans loading gate and keep the BP capability and save money on the purchase to boot. It ain't THAT big a deal to pull the cylinder out at the range for reloading, at least in a Remington. I do it with my NAA mini revolver all the time.
If I didn't already have a bazillion cartridge guns, I'd get a .45 conversion for the Old Army. It's a strong frame and could take some stiff reloads I reckon. However, I own a Blackhawk in .45 already, love the gun, extremely accurate, strong, 4 5/8" barrel for easy carry, and all stainless. I got loads for that thing that equate to .44 magnum, only with a bigger, heavier bullet for field use and a 255 grain cast flat point load for range and general purpose that puts five shots under an inch at 25 yards. It is an impressive revolver I doubt I could match with a Remington conversion.
IOW, if you REALLY want a cartridge gun, buy one. If you just have a bunch of BP stuff and wanna convert one and don't care about BP capability and don't have a good big bore cartridge gun already, you might want the loading gate. If it's just an accessory as it would be for me, go for the drop in. It sorta depends on your situation and what you want, I reckon. I've got single actions in .357 magnum and .45 colt as well as my Old Army and .31 cap and ball guns. I have DA revolvers in two in .22LR, several .38 special, one in .357 magnum. Then there's the autoloaders, several in ..22, .380, two in 9mm, .45ACP. So, you see, I'm pretty flush in the cartridge thing, why I'm not real hot to get a conversion right now. If I got one, it'd just be a neat accessory to my gun, not to take the place of BP. I wouldn't want a permanent mod and I wouldn't tell the ATF about it....shhhhhhhhh!