Really, it boils down to whichever you like best and suits you better. The only advantage of the 56mm objective is that it means you will be able to use the higher powers longer as light fades, assuming equal light transmission, etc. At 3x, both 50mm and 56mm will have an exit pupil larger than your eye can practically use, so really it comes down to whichever sight picture you prefer (colour, edge sharpness, reticles, etc.) Personally I don't have much time for vari-power 56mm scopes and find 50mm about the maximum that balances and fits nicely to a rifle, but that is partly aesthetics and may not bother you.