I expect OW will have the biggest crowd on PC simply because Blizzard has a lot of fans in the PC community due to their limited exposure on consoles - although D3 was a hit on consoles, but even still, I suspect that with such a large part of their fans via PC, it makes sense that OW would be really big there.
Battleborn doesn't have that same PC presence that Blizzard's games do (not meant in a bad way), so I think any of the 3 platforms could potentially have a big presence for Battleborn. I certainly like the performance perks of PC much better, but if the game is not as populous after a while, then I'd hesitate. I'm thinking of games like Star Wars Battlefront where the PC community is quiet comparatively, and obviously the Call of Duty games - consoles seem to be the most active communities with those.
Wasn't there some perks (however minor) exclusive to PS4 as well? Like a unique character if you played in the PS4 beta?
I just wish we could have cross platform play so this wouldn't be a consideration.