Two different cards using the same gp104 chip. Secondly Producing more heat and consuming more power is virtually the same thing in this case (being an EECE major has its' perks lol). As for the rest of your post, refer back to my earlier post in which I addressed your concerns in a somewhat concise manner.
The discussion isn't about Vega though it's about the 980 ti and the 1070.
Obviously bringing AMD into the mix brings a whole other set of variables such as vendor bias and what brand people prefer to own. When it comes to 1070 vs 980Ti though it's pretty obvious the 1070 is the better card but:
Two different cards, but lets go ahead and assume the same thing applies. It boils down to this... If all you're interested in is marginally better performance in what will soon be legacy games, and don't mind paying more for an EOL card that produces more heat, consumes more power and will be slower in multiple other areas like DX12, VR, potentially other API's like Vulkan, and anything that could benefit from an extra 2GB VRAM, and don't mind the very real possibility that driver support will be a 2nd priority compared to the current gen cards, then the 980Ti is the card for you.