Keep your foils, keep your non foils too. I have 10 non foils and 1 foil and will continue collecting them like mad. The card has a good chance of either staying at it's current price or dropping a tiny bit until the block rotates out, at which point it will likely rise in price and continue doing so over the years. If you sell them, years from now you'll be saying stuff like "man I used to have a playset of those, wish I had kept them." I don't think it will ever hit Force of Will prices, but I could see a foil misstepp going for a 80 bucks 4 years from now, and a non foil going for 20 or 30 once the print runs out and they get scarce.
As for Karn, a group of us bought 9 boxes and between us all, I am the only one that pulled a karn. I'm happy with having my one. I like to have at least one of every rare and Karn is pretty damn powerful in EDH.