There's no real reason why you can't be on both sites...however if you only want your bankroll
on one site then it ultimately comes down to what you are looking for. I am active on both and will offer my opinion, for whatever that's worth to you.
Reasons I like PokerStars
- Tournament schedule I feel is more robust
- Special events are more frequent
- More variery in cash games
- Spin n Go traffic is better even at the higher stakes
- Spins are your choice - 25BB or 10BB to start (WSOPca is 15BB at the 2x & 3x multipliers; 5x at higher)
- More opportunities to do challenges for chests & rewards
Reasons I like WSOP.ca
- Rakeback is better than on PokerStars
- Live satellites are available - WSOP ME for example from what I've heard
- Cash game traffic seems to be better at any time, especially in the evening
If there's a particular aspect you're looking for that I can provide more context let me know.