The problem with most of the benchmarks from earlier in this thread, is that they are referring to the difference betwen 680MX and 780M at stock speeds, where the 780M wins by a small margin. However, from what I have read on this forum, it seems 680MX has higher overclocking headroom. So when you factor in overclocking the difference between these cards is almost nonexistant. Remember 680MX is 120W vs 780M 100W, which could be a factor when overclocking. More vram is nice of course, but not essential yet considering the GPU limitations of gaming at 1440p with this generation of mobile GPUs.
The next real GPU upgrade should come 2H this year with Maxwell. Then we should see a fairly big upgrade in performance, hopefully.