.44 Magnum is actually .429"
.41 Magnum is actually .410"
Given the ready availability of .44 ammo, the compatibility of .44 Special ammo with a .44 Magnum revolver or lever gun, and the fact that it's not all that different from .41, I'd say that the .44 Magnum is the most logical next step.
I know the hardcore .41 Magnum fans would prefer the .41, and they may well be right from a strictly technical perspective. The .41 may, in fact, be a better cartridge (like the 6mm Remington, aka .244 Rem, is considered by many to be superior to the very similar .243 Winchester, or the .280 Remington over the similar .270 Winchester). But like the rifle rounds mentioned, the differences between .41 and .44 are most meaningful to skilled handloaders who spend a good amount of time with their cartridges.
If you just want a round in a certain class, and want to make it easy on yourself, get the .44, .243 or .270. If you want the absolute best round, and you enjoy screwing around with all sorts of specialty loadings, and you don't mind committing to loading your own, get the .41, 6mm or .280 (or a heavy gun in .45LC, which can be loaded hot as a .44, with big bullets). If you want an absolute cannon, skip the .44 altogether and get a .454, or the newer .460, .480 or .500.
But the
next most logical step is the .44, all things considered, IMO.
What our wives don't understand is why we need so many guns. They just don't understand how many
calibers there are!