Karo is one of my favorite fighters and I'm not a big fan of Koscheck, but I'm going to have to give Kos the nod in this fight. First, I look back to Karo vs. GSP. GSP was the superior wrestler and the stronger opponent. Because of this, GSP was in very little danger of being submitted (He yanked out of multiple kimuras) and was able to control the fight on the ground. Now, as we know, Kos is one of (If not THE) the best wrestlers in the sport. Now, the Karo vs. GSP fight was several years ago, but I don't know if Karo has the strength or wrestling prowess to control Kos on the ground or avoid takedowns.
Both fighters may decide to respect each other's ground game and keep it standing, in which case I'd give it to Karo, but I can almost guarantee Koscheck will take it to the ground and lay'n'gay his way to victory. Karo will defend himself well from taking too much punishment, but I don't think he'll be able to pull the sub off or slip out of Koscheck's mount.
But as I've said in other threads, I really hope Karo beats him up good. I love Karo and want to see him winning against the top fighters.