I suspect this will pretty much top the BBC viewing figures on Monday, the games between these two are usually good watching. I think Arsenal are probably favourites on form coming into this, because they have been playing with reasonable consistency except for the Monaco game, while Man Utd have been winning ugly (lets be honest they were really lucky against Newcastle). However, it was 2007 the last time Arsenal won at OT, they seem to have a bit of a hex over them in recent years.
I'll go with 1-1, Arsenal to win the replay.