Tough one here as both are good options with favorable match-ups.
I think I would go with McGahee here, as I think RB is a safer play than WR, all other things being equal.
Another concern with Harvin is the fact that Minnesota's passing offense was non-existent last week. It is still difficult to bench a dynamic player like Harvin, but I would give McGahee the slight edge in this one.
Here's mine: viewtopic.php?f=19&t=527243
QB: R Griffin III, A Luck
RB: A Foster, F Gore, T Richardson, K Hunter, M Hardesty, A Green
WR: V Cruz, M Austin, P Garcon, S Moss, B Hartline
TE: J Witten
K: Matt Bryant