a la Pristine Powerstone drop this to uncommon and maybe add a slight cost to untap. "Renewing Powerstone".... ~ doesn't untap during your untap step, you may pay to ignore this effect. ??? Just ideas. Could even drop the enters tapped bit.

See my comment on Worn Powerstone.

I like the "doesn't untap during your untap step, pay to ignore" suggestion. May go something along those lines.

Personally I'm not keen on having cards in the same set that are strictly better than each other, especially at different rarities. Knight of Cliffhaven felt odd to have Glory Seeker in the same set; these feel worse. It'd be much better to have their costs vary as well as their effects. This could still fit with your desired flavour: it makes sense that a non-worn Powerstone would take more mana to summon than a Worn Powerstone.

I think i see what you mean. Hmm, so not strictly better just generally good and useful. I'll have to work on it.