Man United are a far better team than Arsenal at the moment - since Arsenal won the Championship a couple of years ago, the strength of their squad has been depleted, particularly through the somewhat short-sighted sale of Patrick Vieira. Arsenal and Man United contested last season's Cup Final, and although Man United lost on penalties, it was an absolutely farcical result - they had totally outplayed and outperformed a very defensive Arsenal side, who knew they were second best. This season's league table is the best guide to current form, though, and Man United are way above Arsenal. Maybe Arsenal's luck will change when they move to their new stadium, but if Thierry Henry leaves in the summer, which looks very likely, then Arsene Wenger will struggle to rebuild. On the other hand, Man United have had two or three seasons in the (relative) doldrums, and have come through their lean patch, Alex Ferguson having bought wisely (for once!) and with a new crop of youngsters coming through. So, from a neutral observer, Man United are currently the better side. Historically, too, they have won more trophies than Arsenal. Although Liverpool outdo both.
At present, though, the question should be "is it true that Man U is better than Chelsea?", to which the answer would have to be probably not. Money, unfortunately, talks in English football these days, and Chelsea have the funds to buy whoever they want thanks to Roman Abramovich.
The top current English league table is as follows...
1 Chelsea Played 35 points 88
2 Man U Pl 35 Pts 79
3 Liverpool Pl 35 Pts 73
4 Spurs Pl 36 Pts 62
5 Arsenal Pl 35 Pts 58
6 Blackburn Pl 35 Pts 54
(Teams play 38 matches, with three points for a win, and 1 for a draw, so as you can see, Arsenal can finish fourth at best (or could be overtaken by Blackburn, Newcastle and Bolton), whereas Man United (mathematically at least) could still be champions (although they won't be, Chelsea only need one draw from their last three games to clinch it).
Hope this helps...you will get lots of replies from both Man U and Arsenal fans slating each other, but I've tried to be objective.