Don't know. I realise the rational view is to want to win something and not give a fuck about what your rivals are doing, but it's not as simple as that. Rivalry plays a massive part of football, and we do keep tallies of the big trophies, league titles and CL wins, not league and FA cups wins. There is a reason Robbie Fowler held up his hand to Utd fans indicate 5 times while he was playing for Manchester fucking City. Torres the same at Old Trafford when he is neither a Liverpool fan, nor did he play a part in any of the victories. It's the big trophies that ultimately count and it's galling to see Utd win either of them. I care much less about them winning the FA Cup.
If pushed I would probably side on us winning the FA cup, as when it comes to the day, I'd be gutted to see us lose. It's only afterwards - and on reflection - when I see Utd winning the their fourth European Cup that their result would sink in. But, for me anyway, it's not hugely clear cut.