I would likely stick with UA Mileage Plus given the mix of domestic and international and not much paid business class, but AC Aeroplan might offer some options that intrigue you if you fly 100,000 miles plus a year.

UA has most extensive choice of upgrades (points and certificates) but AC likely could accommodate some of your international (likely none domestic though) needs at almost any fare through use of Special System Wide Upgrades (but you don't get many of these in a year and expiry dates are still a challenge -- ones issued in December expire two months later in February (not electronic yet). AC program could get you into UA RCC lounges on domestic flights while you would have to pay with UA's program for domestic flights. Even if you don't fly 100k miles per year, AC will get you this for only 35000 status miles.

For those who fly 100k status miles, AC Aeroplan offers instant kk which means you can get reward seats on AC metal even when reward seats are unavailable for basic points (ie doesn't cost any more than regular awards) provided flight is not oversold.

Other things to consider though--can you get a mileage earning credit card for Air Canada Aeroplan? Definitely get a UA earning one if you go with UA.

If you flew a lot of first or business class, LH and BD are schemes to look at.

See comparison chart below: