You should read replies from this thread. It looks like they would prefer Supercharger.

Here’s explanation why:

Supercharger let’s you continue playing the game with same gameplay as you normally do. You and team just get boosted by it.

Valkyrie takes lot of the Mercy gameplay away. You are the supercharger now. Just hold down mouse button to heal for 15 seconds while floating around. Going battle Mercy is fun part of it, i guess, but doing that means no heals for the team.

Supercharger is not super flashy and cool ult, but at least i can continue playing regular Orisa with more damage. I don’t play Mercy, but i think Mercy mains would prefer something like Supercharger over Valkyrie.