At any price point with mid range cards, you will get similar performance.
It may vary by game, so if you play one game primarily, look for benchmarks with that game.
Tom's gpu hierarchy chart is a useful place to compare(it needs to be updated)
http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
You can't compare specs; the card architectures differ and specs are used to meet a target price/performance level.
The proof is in the performance benchmarks.
And...
VRAM has become a marketing issue.
My understanding is that vram is more of a performance issue than a functional issue.
A game needs to have most of the data in vram that it uses most of the time.
Somewhat like real ram.
If a game needs something not in vram, it needs to get it across the pcie boundary
hopefully from real ram and hopefully not from a hard drive.
It is not informative to know to what level the available vram is filled.
Possibly much of what is there is not needed.
What is not known is the rate of vram exchange.
Vram is managed by the Graphics card driver, so there may be differences in effectiveness between amd and nvidia cards.
Here is an older performance test comparing 2gb with 4gb vram.
http://www.pugetsystems.com/labs/articles/Video-Card-Performance-2GB-vs-4GB-Memory-154/
Spoiler... not a significant difference.
I might add that DX12 is currently a non issue. Games are not using it yet, you need windows 10 for support. It only really impacts the cpu path through the driver, it only improves a relatively minor part of a game workload.
I might guess that the newly announced GTX950 was released to fit in exactly that slot.
GTX960 would be a bit stronger, GTX750ti perhaps a touch weaker.
I doubt there is much difference in driver quality.
One difference is that the nvidia Maxwell cards are more power efficient. They typically need 75w less than the similarly performing amd cards.