The bigger issue in your situation, is that you want an open-ended return ticket. Different airlines will have SIGNIFICANTLY different change policies and fees. Westjet for example, allows you to change up to departure time for $50. Others won't allow you to change at all. Others will allow it, but you will pay a fee that might be as high as the original ticket price...
At the end of the day, it's one short flight from here to LA. All airlines will get you there, probably in a more or less timely fashion. Look on Expedia or someplace for best rates, but then review change policies on the airlines' websites and then book direct.
In the past year, I have flown on Westjet three times (love them), United, US Air and Delta each a couple of times, and once on American. Honestly, I couldn't tell the difference between any of them, other than Westjet... If you are a regular flyer, it makes sense to investigate loyalty programmes (as Guy seems to have done). As for getting bumped, just be sure you check in with plenty of time, and you should be fine.