1. Gigabyte doesn't have the ability to run off the fan at 2D or light loads unless they changed this with a BIOS revision. For that reason MSI Gaming is preferable. Also, MSI Gaming is quieter at load to start with.
2. Gigabyte's warranty starts from the date the card has been manufactured, not from the date you purchased it. Therefore, MSI's 3-year-warranty is actually longer in the real world than Gigabyte's.
3. Check the warranty on both brands as I believe in the 2nd or 3rd year MSI's warranty changes in terms of who pays for shipping costs back to you. I think in the first year they pay for it, but later you might have to pay for return shipping or replacement parts such as fans.
4. Upgrading from an i5 4670K to Skylake for gaming is a waste of $. You are better off grabbing a solid aftermarket cooler like $40 Thermalright True Spirit 140 and overclocking that CPU to 4.5Ghz+. Think about it, the amount of $ you'd sink into Skylake might net you a 3-4% increase in gaming performance with a card of 970 level performance. It's a total waste. BFG tested 66 games and going from an i5 2500K to a 4790K netted just a 6-7% increase in gaming performance. You would be better off overclocking + buying a new $400 14nm GPU unless of course you can resell your parts for 80-90% of the cost or something.
Wow, point #2 is silly for Gigabyte. Thanks, that made the decision for me.
On the last point, there are a couple if issues. The first one is that my board is narrow, so most coolers interfere with my RAM. The second is that the computer is also used for streaming, so the extra threads of an i7 may help. I suppose it would make more sense to upgrade to a 4790K and replace my RAM with low-profile RAM, though. I'm just not sure how well I'll be able to stream newer games with only 4 threads. I'm at 4.2GHz and I think 1.180V right now, btw. I might be able to push the speed farther, but it might not handle Prime95 at that point.