In my opinion the choice depends on what you prefer. Steam is the most established choice and has a huge community, which means more reviews, workshop, mods and generally more user-generated content. Updates are handled automatically and everything works pretty well. Epic has the advantage of weekly free games that are actually worth getting, and sometimes there are interesting discounts. As for future access, both platforms guarantee that your games stay in your library. Neither one will suddenly take away the titles you've bought, even though you should obviously always read the ToS. Basically if you buy a game on Steam today, in 10 years you'll still be able to download it, same goes for Epic.
As for updates there's not much difference: both download and install patches automatically. Steam is a bit faster overall and has more granular options to control them. Epic is improving a lot in this regard. Honestly what I'd do is keep both. It takes 2 minutes to install Epic and you redeem the weekly free games (so over time you build up a nice library without spending anything), but you keep buying on Steam because it has a more mature ecosystem, better sales during discounts and anyway it's the platform you already know. It's not an "either one or the other" scenario, you can totally have both. It's different if you're thinking about places like GOG that have different philosophies, but between Steam and Epic in my opinion having both is the most practical solution.