The inside of Canon hoods have a furry material on them which helps reduce internal reflections. 3rd party copies tend to be made from smooth plastic. It probably isn't significant in 99% of conditions. Also the 3rd party one I got for my 85/1.8 just didn't feel as well made as other genuine ones did, but of course it was a lot cheaper too.
Oh, specifically on the 15-85 hood, I did get the genuine Canon one for mine, and found myself never using it. As with most other lens hoods that cover wide angle and going into tele region, avoiding blocking for the wide angle means it is near useless at preventing flare and offers minimal protection. So I chose not to add the bulk of carrying it around. It's not long, but it is quite wide.
_________________Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.