I don’t think I can choose. I use both, and am good friends with the devs of both options!!!

I use Heroic for my desktop PC, the ease of having Epic, GOG and Heroic on Linux is unbeatable to me. I do also have it on my Steam Deck

Junk-Store is (at the moment, however the upcoming release will change formats) a plugin via Decky Loader, and looks almost as if it is a native extension of the Steam Deck. The UI/UX feels stock, and that is so nice to see. And it is dead easy to use.

JS has Epic for free, and GOG is a once-off payment for the plug-in.

Heroic’s support of Epic, GOG and Amazon are all free.

Its a choice, I swear by these two options and wouldn’t even bother with any other. Try both, and see which one feels best to you!