I'm a little late to the party here, but another thing that is nice about the 2013 Mac Pros compared to the iMacs is that if you opt for a Mac Pro, you can also upgrade the processor to a degree in the future.
You can upgrade the RAM and flash drive, but everything else is soldered to the board... and even it weren't, the only upgrade to the V2 Xeon series are the new V3 versions, which use a different socket with a different number of pins.
I really do wonder when Apple will finally announce a 2015 Mac Pro - the new processors are here, new V-NAND SSDs are here along with the NVME protocol, the USA factory is finally running up to speed, and there are many improvements with which Apple can still wow people with; imagine a "double-height" Mac Pro with dual processors, an extra bank of RAM sockets for up to 128GB or 64GB of cheaper RAM sticks, and support for two SSDs.
Also, I would suggest they drop AMD cards in favor of Nvidia Maxwell cards, which are just as fast, but generate considerably less heat and use less power than anything else. With two cards in a system this isn't an insignificant consideration (up to 300w difference), and could easily open up enough of a power budget to use faster/more components elsewhere.
One can order up a PC that has all of the above today, and perhaps even make it a Hackintosh, making the only difference a pretty exterior and six native thunderbolt ports, although you can buy an add-in card for that as well, so Apple really have to step up their game. Building a closed workstation system is a pretty risky proposition as-is, considering that just about every other company (Dell, HP, Lenovo) only talk about how expandable their systems are. Lenovo in particular has developed an interesting mezzanine connector for the P series that lets you add multiple special adapters internally to their workstations that don't take up any PCI-E slots, yet offer the ability to add m.2 SSDs, RAID controllers and other things, for as many as you need (depending on base model). Also I believe it's one of the most gorgeous PCs ever designed, but that's just my opinion: