Metagross advice

The 98% one is worth taking to level 40 (if you are at least level 38 and gym offence matters in your area).

I would do both honestly it turned out to be that good of a Pokémon.... Power up the ones you have already further and both the ones you are thinking of

Have you run into a situation where you felt that you needed to have another one maxed? You don't need more for duoing Togetic, and for gym offence I'd say 1 maxed is generally plenty enough.

There is no answer that anyone on the internet can give you on this if you don't provide a lot of context to your situation or outline some goals that you want to achieve. If you think you need one more, go for it. If you don't see a need, spend the dust elsewhere.

I'd say power up a lower level one first. You see a much bigger return on investment from that, and then work on powering up the higher level ones to level 40 as time goes on

(btw, got 10 MM metagross, slowly powering each one up here to get the most out of them)