I think manchester United is better at the moment. The players confidence is on the highest level after they won 2:0 against Schalke in Gelsenkirchen in UEFA Champions League semifinal. Not they only win, but they also outplayed Schalke. On other side Arsenal is having big problems in the last period. They lost 1:2 from Bolton last Sunday. They have just one victory in the last seven rounds. I am sure the players are frustrated as this is gonna be another season without a trophy. Even the manager Arsene Wenger is little frustrated by that fact. In the official statement he blame himself for this fact, but that doesn't help much.

To be the situation even worse, Arsenal have poor record against Manchester United in the latest period. Manchester have six victories and one draw in the last seven mutual games. Here are the results:

12.03.2011. Manchester Utd - Arsenal 2:0 - FA Cup
13.12.2010. Manchester Utd - Arsenal 1:0
31.01.2010. Arsenal - Manchester Utd 1:3
29.08.2009. Manchester Utd - Arsenal 2:1
16.05.2009. Manchester Utd - Arsenal 0:0
05.05.2009. Arsenal - Manchester Utd 1:3 - Champions League
29.04.2009. Manchester Utd - Arsenal 1:0 - Champions League

I believe in similar scenario today.