Which Mewtwo should I power up?

Definitely wait. There's not much you can attack best with Ice Beam now, and you will hopefully catch plenty more Mewtwo over the next month.

General rules - only power up something when you have a use for it.
Don't power up something while it's still around, unless either it's perfect IVs, or it's a great counter for itself (like Rayquaza) and you get a top shelf one (maybe not perfect, but close).

Got a 96% Raikou from research tasks (hence level 15)with TS/WC. Didn't get any Raikou when they first came out. Have 30 candy, and will not power it up until either I need it badly for something, or we finish Raikou raid day (fingers crossed it happens) and I don't get a good one, or both.

My very serious player pal caught a perfect Rayquaza on the first raid of the first day it was out. Didn't power it up, later in the month caught a perfect weather buffed Rayquaza, that he powered up.

Patience pays off. Haste makes waste. YMMV.