Sell it on eBay and put the money towards the new one. That is what I am doing!
That's what I've done in the past. Loaded it up with software, sell it before the AppleCare runs out. My 6-Core MacPro5,1 is going on three years, AppleCare runs out next month. Depending on price points and reasonable specs for the new system, it may not pan out as well.
Do you believe people will want current Mac Pro's over the new system, thus driving demand and price online?