Steam are claimed to have 90M active monthly users, I can't find a figure for Epic; it stands to reason it's less though, maybe half their registered users (now Fortnite lost its shine??) would be a reasonable guess?
It probably depends on your demographic considerably; basically if Epic have less than 79M active and the demographics are the same then revenue from Steam will be greater, across a wider user base (which might improve ongoing prospects?).
Your final sentence seems to contradict the rest of the paragraph. If the games are the same price, why are you choosing EGS (to contribute more to the company? but that would be choosing a game store).
Steam's discoverability is pretty piss-poor, and most of the marketing is sales that have convinced your customer base to never buy anything new, just wait for the sale (which cuts into your income with 50%+ discounts).
I'm more bemused by the fanatical loyalty to Steam.
There's a simple solution to that. Just charge more for the game on Steam et al.
For example. Untitled Goose Game is £15.99 on the Epic Game Store, so charge at least £18.39 on Steam.