Given that you will have to collect and recheck your bags on the outbound it might be useful giving WestJet a call in advance to check this. I agree with Redwood, though, that the answer should be BA's allowance.

Notice that as of quite soon WS (and indeed AC) have removed their free bag allowance on domestic economy flights, i.e. unless you're travelling quite soon the difference would be between two or no free bags.

(ETA: of course your OWS standard is not likely to impress anybody at WS since they're not part of oneworld)