Both are comparable cards. You will be hard pressed to see any difference between them without a synthetic FPS benchmark.
And, one might be better than the other, depending on the game.
Ignore the specs, they are all folded into the overall performance metric.
The engineers select those specs to maximize the value at every price point.
The specs apply to different architectures, and are not directly comparable.
For you, I would go with the Nvidia card since you are comfortable with their drivers.
Another possibility which you might consider is consistency of response time.
I recently read this article comparing a GTX660ti and a 7950.
It might also apply to a GTX670 and 7970:http://techreport.com/review/23981/radeon-hd-7950-vs-geforce-gtx-660-ti-revisited
Also, I think that the amount of Vram might be mostly marketing hype.
Read this: http://www.pugetsystems.com/labs/articles/Video-Card-Performance-2GB-vs-4GB-Memory-154