I don't like either of them. I don't want DRM or a proprietary client whatsoever, so I don't use either of their services. That said, I'd rather see Valve do well. At least they've done a lot for GNU/Linux gaming that have indirectly benefited me. Epic has been flat-out spiteful towards Linux. Even if the Epic client works fine under WINE, supporting them would probably actively hurt the future of Linux gaming.
Ideally, I'd rather see GOG step up and start properly supporting Linux, but I don't see that happening anytime soon. Even though I prefer GOG, it's purely because I don't have to run a proprietary client to download and install their DRM-free games. Their Linux support is just sad. If Valve gave us the option to download DRM-free games through our browsers (especially Tor Browser), I'd be all over them.