I'd like to see an Airbus A220 order myself.
Unlikely for a few reasons:
1) Airbus 220 has lots of orders already which means slower delivery time.
2) WestJet has an all Boeing mainline fleet and has never considered any order from Airbus.
3) Embraer 175 could go to Encore.
4) Embraer likely to discount more than Airbus due to less demand.
5) Due to popularity with EMB175 for US regionals possible to pick some used E1 jets from lessors.
In re point 1 that's speculation. Manufacturers are usually able to swap or free up slots for delivery for important customers. A major order from WestJet would probably get earlier slots.
In re pont 2 thats not true. A330s were allegedly almost on the property until Boeing swooped in and dumped 767s for practically free. Further, the A220 is arguably a Bombardier product and WestJet has plenty of those on the property.
In re point 3 that's also not true, Encore is only allowed to fly turboprops due to the pilot/dispatcher union contracts
In re point 4 that's speculation. I think that Airbus would be thrilled to get a substantial order from WestJet and would discount heavily to get their foot in the door.
In re point 5, the E175/70 has been dumped repeatedly due to costs by other airlines such as Aeromexico and Air Canada. Why would WestJet get a more expensive plane that everyone else rejected. That's just going down the 767 route again.
If WestJet gets an Embraer I think it would be more likely to get the E2, but again since they sell so poorly that doesn't seem like a good investment to me for resale/lease terms. I don't know that the A220 is substantially better, but I think with the support of Airbus it is more likely to have better long term value.
I'd say the A220 at WestJet is more likely than an Embraer, but would put the odds at 60/40 in favour of the A220.