The 1060 is 5 ranks higher on the GPU Hierarchy list when compared to the GTX 750 Ti. The 1060 3GB would offer near the same performance of the 1060 6GB, but Nvidia shut off 128 cuda cores or 1 SM. They did this so it wouldn't perform at 1080p. You can watch the video at the bottom.
http://www.tomshardware.com/reviews/gpu-hierarchy,4388.html
"Imagine this with one less SM, and you've got the GTX 1060 3GB."
"Is the GTX 1060 6GB Worth It?
This comes down to a question of whether the difference is ever noticeable in gameplay, and if it's worth the $50 extra for the 6GB card. We know that overclocking would scale performance mostly linearly, especially shown by testing the cards clock-for-clock, and that the performance deltas can be mapped to the reduction in CUDA cores and VRAM. For most games, the difference is about 6 to 7%, but we're seeing huge swings in some cases. Black Ops III posts a 10% performance change going from the 6GB card to the 3GB card ($200), Shadow of Mordor shows about a 14% change, and Doom is nearly 0 with OpenGL, but posts massive, 27% differences with Vulkan.
And then there's Mirror's Edge Catalyst, where we saw a performance drop from 58.3FPS AVG / 39.3FPS 0.1% low with the 6GB card, the 3GB model falling at 33FPS AVG / 22.7FPS 0.1% low. That pushes us below the “playable” threshold for Mirror's Edge Catalyst at Hyper, which wasn't the case for the 1060 6GB unit. A purchaser of the 3GB card would have to drop settings to Ultra (1080p) to remain playable on the unit, whereas the 6GB owner could play at Hyper"
If you are gaming at 1080p the 3GB it will do fine. The 6GB is a little better in some games.
http://www.gamersnexus.net/hwreviews/2604-gtx-1060-3gb-vs-6gb-benchmark-review?showall=1
The Video
https://youtu.be/zpb6qaH_saQ