Hey, it's me again. I'll be expanding my reasoning and comparison between these two cards.
But first, what do you plan on doing?
This 1080 card is great for top latest games and 4K. It's pretty much about 10% improvement compared to 980TI and 40% for 980. 1080 is a direct replacement for the 980/980.
You could save your money and wait for couple years and whatnot for a better GPU, or buy the 1080 to be pretty future proof, since the 1080 had the fastest processing power compared to most other GPUs.
In some games there isn't really much performance boost, but some other you can see about 40 or more FPS improvement on ultra settings. It depends.
Honestly it's all up to you. Many people are switching over to GTX1060-1080, but if you really want those little extra boost and future proof experience, I guess you can buy it.
I'd get it if I really want just 4K gaming for next few years, but keep the 980TI If you really just don't care about the 4K aspect. However if you really want more FPS, then 1080 is the way to go. But it depends.
I can elaborate, but honestly. You can do your own research and comparison and watch some benchmarking videos.
I've been reading and hearing that it's he best GPU out there, but yeah. That's all I really got to say now.
Hope this helps a bit. Any question and let me know any time!
Btw: VRAM doesn't really matter. 6 GB is good enough, by if you just really want to be future proof and get a tiny bit more Fps, I guess you can go for 8GB for like 4K gameplay and whatever. More explosions and stuff going on screen I guess.
*two thumbs up*