What strategy should I be aiming for with decorations?

I'm at the point where I've unlocked all 14. They're all at least level 11, and I'm getting 20k Shares for selling the company each time.

Level cost reduction seems the best, but at level 15 it costs 23.82k to upgrade.
Increasing shares seems second best, but it's also at level 15 and costs the same 23.82k to upgrade.

Should I just keep grinding at those, or is there something else I should be focusing on?