SLI Setup for GTX 780 SC or GTX 970?

Hey Tom's Hardware Folks,

I need some advice on what to do about my graphics card situation. Right now, my setup is as follows:

-Corsair Carbide 500R (http://www.newegg.com/Product/Product.aspx?Item=N82E16811139010)
-ASUS Z87 Pro (http://www.newegg.com/Product/Product.aspx?Item=N82E16813131979)
-i5-4670K (http://www.newegg.com/Product/Product.aspx?Item=N82E16819116899)
-EVGA Superclocked GTX 780 3GB (http://www.newegg.com/Product/Product.aspx?Item=N82E16814130917)
-Corsair HX850 850w (http://www.newegg.com/Product/Product.aspx?Item=N82E16817139011)
-Corsair Vengeance LP 16GB (2 x 8GB) (http://www.newegg.com/Product/Product.aspx?Item=N82E16820233280)
-Cooler Master Hyper 212 EVO (http://www.newegg.com/Product/Product.aspx?Item=N82E16835103099)
-Samsung 840 EVO 120GB (http://www.newegg.com/Product/Product.aspx?Item=N82E16820147247)
-Seagate Barracuda ST2000DM001 2TB (http://www.newegg.com/Product/Product.aspx?Item=N82E16822148834)
-LG 12x BD-ROM (http://www.newegg.com/Product/Product.aspx?Item=N82E16827136268)
-4 x Rosewill 140mm fans (http://www.newegg.com/Product/Product.aspx?Item=N82E16835200067)

I built my rig planning to go to an SLI setup as soon as I saved up enough money for the second graphics card. I decided to wait for the next promotion on the above listed GPU before I was going to order it but low-and-behold, they discontinued the model before I got a chance to order it. So the options I'm considering are as so:

1. I order another EVGA Superclocked GTX 780 on eBay (I'm seeing these go for around $300 to $350 since the only new models I've found have a ridiculously inflated price of around $600 to $800)

or

2. Sell my own EVGA Superclocked GTX 780 on eBay (to make around $300) then put the money I earn towards 2 GTX 970s. The problem I'm seeing however is that there seem to be a wide range of 970s available. The benefit to this is that I have the benefit of having two brand new cards, an overall higher grahics performance, plus with Ubisoft's 1 free game with purchase promo going on right now, that's two games I could pickup for free (Assassnin's Creed Unity and Farcry 4).

Here are my questions:
1. What are everyone's thoughts on the best solution?

2. Are there any specific models of GPUs that would be good with an SLI setup? I'd prefer to stick to EVGA or ASUS brands as I've had good experiences with those brands in the past. I know that the fan setup on the GTX 780 I have now works with an SLI setup but I'm not exactly sure how to tell the difference (a fellow Tom's Hardware person helped me find the right setup).

Thanks in advance for everyone's help.