On the other hand it depends which components will need to be upgraded, like CPU or GPU options may be limited and possibly pricey. The mini could have better resale value in a year too, so upgrading the machine (by swapping it entirely) may not be a huge financial hit.And in a year when the Mini is considered slow by the newer standards, you will spend more money replacing it than you would by just upgrading components in the Mac Pro. Just something to think about.
However if the smaller form factor of the Mac Mini is more important to you, then the mini is a great choice.
The form factor becomes a matter if you want to keep the old machine around. Mac minis can pretty much be used for anything and be discreet about it (like HTPC, file server, etc) since they're so compact...not so much with Mac Pros. They're just excessive both size and power wise for secondary use.
Belkin should make a new version of this thing to fit the new model.Go for the mini its a little powerhouse of a machine. The only thing I wish it had are 2 front usb ports.