Evolve or power up?

Can always use rare candy later to power up; can only get Meteor Mash now.

One maxed is enough just for fighting gyms right now.

For raids, you will probably want more. For example, when Togetic was briefly a raid boss, I think* two players with three level ~30 Metagross each were able to beat Togetic.

*I had four Metagross (levels 35, 32, 30, 30) and was able to duo Togetic a few times. (My partner had three Metagross with similar levels and IVs.) I usually only needed three of my Metagross, but one time (maybe against Dazzling Gleam?) I dipped into my fourth. Three Metagross and a Raikou should probably work too.

Additionally, you might want to only power the little ones up enough to use them in the GL or UL (1500 or 2500, respectively), in which case they won't need much candy at all to power up. Actually, you might be too strong for the GL with these, check using the CP calculator.

I'd evolve both, you will always be able to buddy metagross afterwards, trade any beldum you catch or hatch for more candy, but we dont know when or if meteor mash will ever come back. Also it isnt super critical to have numerous high leveled meteor mash metagross for raids at the moment one maxed good one will more than suffice for all your gym crushing needs.