Shiklin :
Would you say the extra 3gb is worth the £40 on the 1060? Not entirely sure about what the extra memory does :')
Short answer is it will scale better with resolution. That means, if you ever decide to go into 1440p (2560x1440) or above, then the extra memory will help the 6GB to have more stuff as in textures and extra triangles (simplification, lol). Plus, the 6GB is a tad better chip wise. For some stupid reason nVidia decided to call 2 different chips under the same consumer name.
If you have the extra money, then by all means go for the 1060 6GB. If you want a compromise, then the RX480 4GB (if you can find one with a decent cooler) is not a bad choice at all.
EDIT: A friend just got
this one and it's one capable card. Not the cheapest, but the value is great IMO.
I have
this RX480.
Cheers!