I lean more towards the "tough bullet" school of thought when it comes to fur hunting.
I usually use 55g Sierra PSPs loaded up to 2600-2800fps in the 223 for fur. They nearly always exit(even on big bobcats), but the exit is usually no bigger than a nickel. I get similar results with 50g Sierra SMPs loaded to around 2400-2500fps in my Hornets.
The Balistic-tips, and other polymer tipped bullets usually blow a fox apart in a 223 at nearly any velocity. Foxes have very thin hides, and if you don't hit solid bone Balistic-tips can make a bad mess out of a good hide.