Used Mac Pro vs tricked out Mac Mini i7

I would go with the Mini in your case.  The only thing the MacPro has is internal hard drive storage.  You can add on as much external storage as you need with the Mini.  As far as the graphics go, I really do not think you will see much difference.  I have a 2012 Mini and it is MUCH faster than my 2008 MacPro with an upgraded video card when it comes to photos.  I LOVE the Mini and you will not regret it!  You cal always sell it and upgrade after a couple of years.

Kevin