Both fests have a great lineup, but I personally like Virgin's a little better. I will say that DAFT PUNK, who is headlining Lollapalooza this year, IS AMAZING LIVE. I saw them at Coachella last year. IF YOU'RE IN THE CHICAGO AREA, GO TO LOLLA! You don't wanna miss those guys... much less Pearl Jam, Muse, Yeah Yeah Yeahs, the Roots, Modest Mouse, etc etc etc.
A couple of bootleg clips for your enjoyment
http://www.youtube.com/watch?v=dmF6wzQc8Og
http://www.youtube.com/watch?v=nZxJ-W8Po_E