I suggest you go for AC

Unlike UA's 1K, as a Super Élite at AC you start the year with 8 sytemwide upgrades, confirmable at res, good from most economy fares (not Q or L, the deep discount and seat sale fares). Further, you get two more for every 10,000 miles you fly (which is another pair for every transatlantic journey).

You could probably get AC to comp you to Élite based on the amount of flying you do with BA. But you would have to get through your first 100,000 miles to get the SE benefits (but that is the same on UA, or just about anyone else).

Downside: If your travels go mostly to the US, you have to connect through one of the Canadian airports. But most have US preclearance stations, which mean that you have less aggro than you would arriving direct.

If you are doing US to Asia, YVR has sterile transit (no Canada customs clearance required) on AC/CP flights to the major Asian destinations.