Both are excellent, and probably up there in the top 3 GTX 670s right now including the EVGA FTW edition.
I own the Gigabyte card myself, but if the ASUS DC2 TOP card was available at launch, I probably would have paid the extra $30 for it. Here's why ...
1) I'm an ASUS fan. I've always liked their products.
2) The TOP version is OCd higher out of the box, and may have more headroom to push higher clocks on average than the Gigabyte card ... in other words, possibly a better chance at getting a good overclocker.
3) The ASUS card looks better and has a nice black backplate.
Both are quiet. Both run cool. Both are awesome. Whichever way you go I predict you'll be more than satisfied with your purchase.