Honestly, they're the same, or darn close. Pick whichever looks 'cooler' to you. I have the Gigabyte Gaming OC model.
1)The performance differences between the 2 are going to be irrelevant.
2)Nvidia's gpus can overclock without our... 'interference'. All you need to do is ensure the gpu gets adequate airflow and stays cool.
3)Good case cooling is more important for these cards than wanting to overclock, as their Gpu Boost algorithm is sensitive to temperature - so much so, that it has a few temperature thresholds. The hotter it runs, the less it'll boost. The first threshold starts around 60C, IIRC.
A manual OC will not fix this. It will still downclock if it feels cooling isn't adequate.
4)Only the Power Limit, Temp Limit, and Memory Clock is worth tweaking, as they're the only settings the gpu doesn't adjust on it's own.
5)The best overclockable gpu is one with a full cover waterblock + radiator on it. Seriously. That's the one part in the PC that sees the most gain from liquid cooling.
I'm aware you're only interested in 2 gpu models, but I chose to throw this tip in anyway.