The 680 is still pretty fast for today, I'd say it's about equal to 960. The trend is NVIDIA likes to keep it that way. 680, one step down to 770 and 960 (regarding performance), it will be significantly faster than the 950. Newer architecture doesn't give you that much benefit, usually, the only meaningful benefit is probably power consumption and heat. Sometimes, new features too but I can't recall exactly what.
The 680 is already 4 years old, that's completely something to consider because there are going to be a significant chance that the video card is going to stop working sometimes in the near future. If I were you, I would decide by price. If that 680 costs less than say, $130, I'll take it. Higher than that, I'd choose the 950. That's what I'm going to do if I were you.