Mini. Comes with 1 year warranty, add the AppleCare and you won't have to spend any more $$ for at least 3 years, except for optional upgrades. Buy a nearly 6 year old Mac Pro, and figure on at least one major repair bill in the next 3 years.
Plus.... in a couple of years the Mini will still be worth some $$ if you decide to sell it and upgrade the system. The Mac Pro is not going to be worth nearly as much in a couple of years.
But a refurbished Mini, and in a couple of years and with a little luck you'll get 90% of your purchase price back. If you are aiming for a Mac Pro, then just make sure all of the peripherals for the Mini can be transferred over to the Mac Pro. It's what I did. I bought a refurbished Mini, base level specs, and ran that for a couple of years. But it started to push some serious peripherals as I added equipment. When I finally started bogging down the Mini beyond my patience, I shopped for Mac Pro - got a good deal on one - and didn't have to spend a single $ more since everything attached to the Mini had actually been purchased with a Mac Pro in mind.
The Mini was retired to jukebox duty, and still runs to this day.
Luck.