I've done several AA segments in the last year as a BA gold. Been upgraded once and it was when they were trying to free space in Economy to JFK as LGA was screwed and they wanted to get people on the JFK flight instead.
Lounge access is worth more to me than the 'first' class upgrades on AA.
But the redemption side of things is worth considering too.
If you are in the AA program - you have access to all levels of redemption - and their redemption availability is good. But as a BA member - you only get the lowest level - which sells out quick.
So it's swings and roundabouts.