Which Golem to power up?

Neither, wait for a good Rhydon

The 98% could be a good defender, but he's tier 3, everyone has vaporeon. With moves of the same type golem could be a niche attacker but Rhydon does exactly what he does but better

I think that the special thing on Golem is that he has a rock type quick move, which is very rare in a final evolution. Correct me if I'm wrong, but I believe that only Onix can learn this move as well. Apart from that, Golem's base stats are very nice, inferior to Rhydon's yes, but he has the advantage of the moveset, I imagine that a 80%+ Golem with Rock Throw / Stone Edge would rock in any gym battles in spite of Vaporeonsssss.

Neither. I would only power up a golem or Rhydon if it's 98% IV or better with best move sets and neither of them are that.