UA 772 the "worst C class cabin in the world"? Oh please. Not even close. At least it's all lie-flats, unlike the majority of the LH fleet. If you can get a good seat (like row 6 on the sides) it's a pretty nice ride. I believe that's the aircraft OP would see from IAD-DXB.

Note also that there is no such thing as an "Old" CO 763 in the UA fleet. CO did not fly 763s, any 763 in the fleet is a pmUA aircraft. There are pmCO 764s. I don't much care for that configuration, it's by far the shortest business class seat in the fleet.

To get back on topic, while the benefit of UA is no fuel surcharges, the downside (apart from crappy food and uneven service) is you would have to clear customs and immigration if transiting back through the US. But UA would not be a bad option for your outbound: YYZ-IAD-DXB. I think I would look hardest at TK/OS/SN for the return. LX would also be nice, but the award inventory releases are few and far between.