Need help with Franken Rig

So recently I have been considering a dual socket Supermicro X8DTI-F and two Xeon X5650s. On eBay this goes for around $300 and offers me 24 threads! However a 990fx board and an FX 8350 goes for around the same. An X79 I7 3820 and EVGA classified is roughly $300-350. Which of the builds is the best? I want to do SLI with two GTX 970s. It needs to last around 6-8 years as well.

What is this rig going to be for? Gaming? Productivity? A mixture of both?

A mixture of both. 

I would also like to mention that this is a project that will take me a while (six-eight months) as right now I'm working on an old Dell Precision 690/T7400.

I'd like to add a third option, and was wondering if it is justifiable: an Msi SLI Plus and a xeon e5-2603 v3 for stringm around $400-450.