More memory doesn't increase performance considerably. Only increases it significantly, and I use that term lightly, if you are playing at a resolution much higher than 1980x1200 and/or if you have two monitors.
So if you play at 1980x1200 or anything below that, more than 1gb wont be necessary.
And to answer the question, if you play at 1980x1200, the 1GB at 950 MHz will suit you better, and if you have two monitors at that resolution, or a resolution thats higher, the 2GB will be better.
The 1Gb 950MHz Card is certainly going to be better.
Reasons, well firstly it's faster, and secondly there is not going to be a texture or bitmap size of 1GB it'll all be in small parts, thus, faster in and faster out will depend on the speed of the processor and the GPU chip.
So , even when it comes to HD gaming the Core Speed is what comes first and then the Memory Size.