I really hit the Beldum CD hard, but this week I’ve got as many extra MM Metagross, mainly shadow but also a non-shadow, as I can and my logic is this:
I only really play PvE
I would never use an ETM/RC on PvP mons because the meta is diverse and changes so often, I rarely use them on non- legendaries at all as there’s always more candy sooner or later.
It’s the best of It’s type by a fair way
There is not much in the future, especially the near future, that will outclass it, especially by any great amount.
Who knows when, or if, the next event to evolve to MM will come around?
Beldum does hatch from 10k eggs and Metagross does appear as a raid boss so there will be a slow and steady candy source to power them up if you don’t want to use your RC.
As @stativision rightly says, Metagross doesn’t see that much outright use but it’s not so dissimilar to Psystrike Mewtwo in that regard. Psychic type isn’t that useful either but when they do, they both shine. Shadow MM Metagross is a beast when it has the chance, yes it’s less common than when, say, fighting is SE but it will serve you well on those occasions and as it’s fairly future proof it will keep serving you.
I believe Metagross is ok in UL but there are also, I think, better options. There are also about 6 billion people on the planet better qualified to give PvP advice than me so make of that what you will.
I’d personally go for the extra MM Metagross now because you can power it up later, whereas the commodities to get a second later, whether 18 months of time for the next event or an ECTM, are more precious and less flexible.
As ever, it’s your team, your resources that you’ve worked hard for and so, go for whatever you think will see you enjoy it most, will get the best use and will make you happy you invested in it.