Should I evolve or not (98%IV Scyther)

I would evolve it. Scizor with bug moves is one of the counters for Mewtwo. So then that pokemon will have a use for you. (Even though its kinda specific)
And its almost perfect, would be a shame to just use it as a trophee imo.

I have an 89% Scyther with Fury Cutter and Bug Buzz and it's my favorite Pokémon so I dediced to max it out. I also have a 95% Scizor with Fury and X scissor

As the moveset SW/BB doesn't hold much use, you could wait for that to change, or evolve to a more powerful beast in hope it gets FuryCutter or BugBite/X-Scissor. If it does then power up if you have plenty of dust..

In that league Pinsir is a bit more robust against Mewtwo.

Not evolving a pokemon like that seems like a waste of its potential. Especially considering how rare it is and how useful it's gonna be. I would definitely evolve it (fun fact - in almost a year of playing the best scyther I managed to get was only 89%).

Worth at least considering: The bug/flying typing has different type advantage against Mewtwo with Focus Blast. If you don't evolve it, you would want to use fast TMs on it to get Fury Cutter.