from your choices the GTX 970 and 770 will be great for your task, if you have the money. but you can get an earlier model, since you dont need lots of GPU power. But in my case ill go with the 970, just to ave something powerful there 😉
is there any Nvidia alternative?
since I will probably use Premiere for editing, and I heard that AMD card may not be that good for those Adobe softwares.
I'm putting an ASUS 770 in my next build (LAN party build) and I've looked around, it seems like a very good card, maybe two of those in SLI would be better than a 970, I haven't seen anything about the 970 yet, price is a key factor, the 770 should be fine with your rendering.
I use a AMD (Sapphire Dual-X) R9 270 and it renders the videos fast, but I'm not sure how long your doing these clips for.
from your choices the GTX 970 and 770 will be great for your task, if you have the money. but you can get an earlier model, since you dont need lots of GPU power. But in my case ill go with the 970, just to ave something powerful there 😉