If you can take the pounding and can afford (and find) slugs for target shooting with your 20, knock yourself out(!) On the other hand, a 30/30, or lots of other CF cartridges, are extremely useful and versatile, and easily handloaded for economy and performance tailored to the use to which you want to put them.
If you don’t handload, right now the 6mm Creedmoor or one of the elephant rounds seems to be what’s readily available in factory ammunition.
