Here in the UK a good vega 64 is about the same price as a 1080 ti armor, and that's if you can find the vega 64 in stock. Also, the gamersnexus video shows the card with 213% power target, which means that it is pulling about 550w alone, and as such really shouldn't be considered as the parts don't withstant such power draw easily. It's pointless to say "But in the US the price is x", as I (and a lot of other people) don't live in the US, and prices in Europe are generally different, and over here Nvidia is a way better deal (I'm not a fanboy I just want the best performance for the best price I can get)
For Example:
Asus Strix RX 580:£270 Asus Strix GTX 1060:£280 (Nvidia has lower power draw and slightly better performance)
Asus Strix RX Vega 56:£400 MSI Gaming GTX 1070 Ti:£405 (1070 ti is faster)
Asus Strix RX Vega 64:£460 GIgabyte G1 GTX 1080:£480 (GTX 1080 is slightly faster)
"Asus Strix RX 580:£270 Asus Strix GTX 1060:£280 (Nvidia has lower power draw and slightly better performance)"
Wrong, the RX 580 has better performance. TechSpot did an article on this recently.
"Asus Strix RX Vega 56:£400 MSI Gaming GTX 1070 Ti:£405 (1070 ti is faster)"
Nope, the two are about even. The Vega 56 definitely has more overclocking headroom.
"And this is without even mentioning the used market where you can get a good 1080 ti for about £430, which is way faster than the vega 64 or the gtx 1080, but still uses less power than the AMD card."
Yes and I can get a used Vega 64 for $240. You can't cherry pick used pricing vs new.
FYI UK pricing doesn't make US pricing irrelevant. The world doesn't revolve around the UK. If anything the US pricing means much more because the market is multiple times the size of the UK.
"I'm not a fanboy I just want the best performance for the best price I can get"
Your comment suggests otherwise. The GTX 1060 and RX 580 / Vega 56 and 1070 Ti are closely matched yet you dained to give Nvidia the advantage decisively to a single brand. TechSpot recommends the RX 580 over the 1060.
Here's value charts from techspot:
https://www.techspot.com/article/1729-graphics-card-pricing-q4-2018/
But according to you Nvidia is the obviously "superior" choice, despite reviewers paining a very tight picture. That to me doesn't sound like objective thinking. It sounds very subjective.
You are getting a similar experience on either a GTX 1060 6GB and an RX 580 8GB. A nice bonus for the AMD cards it that right now they come with a free game bundle and you can get FreeSync on the cheap (free in most cases). The Nvidia variant will run you $200 more on average. That's a big deal given that's almost the price of the cards themselves.