In the broadest possible terms, a 2nd in the TDF is bigger than a P-R win. That said, they are totally different types of race. No one who gets close to GC in TDF will ever win P-R. I think a better question would be Green Jersey vs. P-R.
I respect O'Grady more as a rider than Cadel. Cadel rides smart, but O'Grady rides hard; Cadel can climb and TT better than all but a handful, which is where the TDF is won, but O'Grady isn't a prick. O'Grady is one of the hardmen of the peleton and I just respect that much more than a top tier GC guy. O'Grady, to boot seems like a really nice guy; I saw him hang around for 3hrs signing autographs and talking to kids at one of the first Tour Down Unders.
I also think that Paris-Roubaix tops any stage win on a grand tour, FWIW, except the gutsiest of solo breakaways.