The 980ti is a good card, but the 1070 is often cheaper and better, the current problem is that bitcoin mining just blew up again so the price of the 1070 started rising, the 1070 is still very affected by this, so i would recommend you to wait 1-3 months and then making your purchase, the price is definitely going back to normal pretty soon.
https://camelcamelcamel.com/MSI-GTX-1070-GAMING-8G/product/B01GXOX3SW?context=search
https://camelcamelcamel.com/MSI-GAMING-GTX-980TI-6G/product/B00ZPEAFXI?context=search
The 2 cards perform pretty equal, but you can oc the 1070 more than the 980ti, this of course depends on the cooling of the card, but speaking in general the 1070 is better for overclocking.
Also the 1070 is newer and is going to be supported in longer time than the old 980ti.
So here's the thing, the 1070 will probably drop to around 450$ in 3 months, and if it drops to that price i would go for the 1070, but if you have to buy it right now then i would go for the 980ti deal that you got.
Hope this answers your quesiton.