Alaska is heavily pushing to expand Horizon's network, and will almost certainly pick the E175 pending discussions with their union (the E175 is also currently out of scope for Horizon) -- assuming of course a workable deal with Embraer.
The CSeries would be a long shot for Alaska. First of all, Alaska needs replacements on routes currently operated by Q400s, so it's doubtful the economics would support even a CS100. Secondly they already have E175s in the network operated by SkyWest.
Also they want planes starting next year and must have them by 2018 since many of the Q400 leases are ending. I doubt they'll want to risk waiting on an aircraft that has yet entered into service.
I'm sure Bombardier is continuing CSeries discussions with Alaska but here they have to downmarket the aircraft and compete against regional jets.