After hearing your opinions, I thought I'd share my own.
I think I'd rather take the FA Cup, for a number of reasons:
1. Guaranteed silverware, which always gets the fans buzzing. It seems to also be our way of 'beating' Arsenal, since we never place higher than them in the league table; we need to win trophies while they don't. 8-|
2. Admittedly, if we were to finish in the top four, who realistically thinks we have a chance of doing well for our first foray into the Champion's League? I'm not sure if just qualifying and getting knocked out early is worth the sacrifice of a trophy win.
3. Finishing in the top four does indeed mean a large amount of revenue would flood into the club that year... but I wouldn't like to get reliant on that money unless we were guaranteed top four each season; which we are definitely not.
4. As has been mentioned, winning the FA Cup means entry into the Europa League, which we have a far greater chance of competing in than the Champion's League at the moment.
Don't get me wrong; I would love to see either of them happen. I just think the cup win would be something that would be talked about for longer than getting into the Champion's League and dropping out early on.