I'm enchanting my pure mage character's boots.

I can only place two enchants out of fire/shock/frost resistance. I was thinking fire and frost resistance since (I think?) they're more common damage types, but I'm not sure. Which two types do you guys find to be more common?

If it's for a mage, I would go frost and shock. It'll help "divert" the loss of magicka and stamina as well. Fire you can put elsewhere, shield (if used) and I believe ring can carry fire abatement.

I'm not using shield, and ring/neck will be used for reducing spell costs, so it comes down to boots only, I'll have to choose two out of the three.

Does Resist Shock also help negate the damage magicka effect of shock spells targetted on me?

If you lose less health from the shock attack, you'll lose less magicka as well as the two are equlvalent.