Canada was looking for a good dual engine fighter due to it's massive size and sparse population, especially in the north. Unfortunately there were no real options that filled our needs. The F 35 is more of a compromise choice. Still, our air force is outdated so needed to happen.
I'd say allied interoperability was the deciding factor.
Now, 4 of the 5 Eyes states (i.e., US, UK, Australia, Canada) will operate the F-35A. For Canada, there's a guarantee now that it can readily deploy its fighters to any major allied operation (i.e., involving the 5 Eyes). In one stroke, Canada can station its fighters overseas without worrying as much about logistics and support (as it would have if it bought a more niche design). I reckon the RCAF will be a common sight again in the U.K., Germany, Poland, and possibly Japan or South Korea.
The other aspect is that with a total order of 2,000+ units, the F-35A offers the most economies of scale. The long-term operating cost is probably projected to be a lot more competitive thanks to the presence of more producers in the supply channel. It wouldn't surprise me if Canada adds on to its current commitment -- I can see them push the F-35 fleet to 120-140.
It sounds aggressive, but I feel that Ottawa has a more assertive defence vision. It's not just about supporting the U.S. or NATO, but also guarding Canada's northern interests. The ice is melting away to reveal new trade routes, fisheries, and possibly even natural resource deposits. IMO, among at least the Conservatives and even Liberals, there's a cross-partisan agreement that those northern interests matter, and they'll need the "big guns." The CSC frigate, the Aurora-replacement RFI, and F-35A all, IMO, speak to a desire to both deter and project.
The last 3-4 years have shown that you can't take free trade, alliances, etc, for granted. However, being one of the world's largest economies, Canada would have a lot to lose if it can't do its part to rein in a major strategic situation. So, the defence capability is about to catch up to the economic weight. The last 'frontier' now is to create consensus for a SSN/SSGN.
@SQ8 @JamD