What it all boils down to is how much you enjoy Barbarian Assault, and how much you loathe training agility. Personally I used to the penance horn for a few mining levels. I found it really helped because mining is so boring, but I really enjoyed barbarian assault, so even if it resulted in less xp/hour I found it much easier to do. On top of that, when you see yourself gaining double exp as you mine, it's easier to push yourself to mine more, in the sense that it'd be easier to make 50k super strength potions than 50k attack potions, even though the actions themselves are the same, you feel yourself getting a better pay out so you're more willing to do it.