Obviously 1060 6G is better choice

Why not upgrade to RX 580 4GB? or cheaper, the RX 570 4GB? Because you're already using high power requirement GPU, so it means that your PSU supports these GPUs.

1050Ti or GTX 1060 is a more power efficient GPU, so it's more recommended on lower spec PSU for the price, because it sacrifices some performance as a trade.

I see, the performance is similar on most cases for RX 580 and GTX 1060 6GB, and price don't differ much to over here. But the 4GB should mostly sell at a lower priced compared to the 1060 though. And then, there is the new RX 570 which mostly performs similar to RX 480 but is sold at a lower price. This should be around the performance of GTX 1060 3GB.

The reason I'm recommending Radeon is because you can utilize your already pretty beefy PSU, with lower price. Else, you may try to find if there is any used 980 Ti sold around new GTX 1060 6GB price.