I have done 8 segments on AC in the last 2 years and each was late - an even worse performance than BA! But earning UA miles is more useful to me, even before the BAC changes, so I would personally lean toward AC, depending on my year end status position.

More minor but LHR T2 is much nicer than T5 and both the UA and SQ lounges are better than even the BA First Lounge.