This is a hot topic that has been tossed around for a while.
EPS has been making exclusivity deals with many developers, keeping their games away from Steam. This practice has been questionable to many people, especially when the deal happens to a Kickstarter campaigns without giving any notice to its backers. Furthermore, people have pointed out that Epic Games doesn't keep a tight lid with their security with their customer base. Their platform also doesn't have as many features as Steam. EPS hasn't been out for long so there may be some improvements to look forward to.
Steam is not without any fault to say the least. Valve used to make great games before being complacent Steam being one of the most dominant seller in the market. Additionally, their lack of curation has caused the store to be saturated with shovelware. They also give developers a smaller cut of their sales on games compared to EPS.
At the moment, I prefer Steam over EPS but I'm rooting for EPS to grow so it can drive some competition. Maybe it'll make Valve develop games again but that's probably just a pipe dream.
I currently only use Steam myself, besides on the rare occasion the game I'm after is only available on Origin, Epic Games Store, etc. That said, I wouldn't mind some competition either, it's healthy for businesses and keeps things evolving and moving forward with consumer demands. Plus, more competition usually means a more competitive market where we're able to get more bang for our buck.
I'm wary about EA Origins. My personal experience with that platform was initially fine until I took a hiatus. When I came back, I found out that my entirely library was gone and customer service wasn't much help with retrieving my games back.
Additionally, yes, competition is great for the market but I'm very concerned how EGS is introducing itself. Tim Sweeney hasn't been exactly graceful on Twitter when responding to topics about the store plus Epic Games hasn't been kind to it's customer base.