Hold your stardust?
For all those asking about who to power up, especially in preparation of legendary pokemon, I say hold off on all for now.
We don't know what the pokemon fest will bring. I have seen a couple rumors pointing to Gen 3. If this is the case you may have a better option, especially for those who have been saving rare candy. We might not even get legendary pokemon.
I say wait and see. You would still have the option of powering up you double rock Golem if the legendary birds come. You may want to use the precious stardust to build up a gen 3 beast.
Answers
That being said, Golem is still a fine choice vs other raid boses, so it's not a bad choice, even when gen 3 is released, he'll not be a bad choice, as the only pokemon to rival him at that point will be Aggron, and they'll probably be about equal (Almost the same attack stats) and we don't even know thich attacks Aggron will get, so it's not worth speculating about it.
Having said that, if you don't currently need a golem, you might as well wait and see what we get from the event.
Wait for gen10 until you start powering up! Honestly...a friend of mine is always waiting and accumulating over a million of stardust each time before a big update and in the end he is powering up the same mons as I do...so I prefer having fun when I feel like powering what I want. It's still a game...
