Both 8gb.....difference in performance and price (Canadian prices! CAD). Yes, also used because....in Canada...! LOL!
Here's a 'for-instance' or example:
GTX 1070, 8gb - $350 (I missed out on one for $300 - it was snatched up quick)
RX 580 8gb - mostly looking at the Nitro+ - I think that is the top 580? - for $245-ish - some are $258 (ebay)
That's the price difference. I guess most of you know the performance difference. I know the 1070 is 'better' but I also use Linux and might try Hackintosh - the RX 580 is supported, natively - so, without too much configuration or tweaking - should 'work out of the box'
The Nvidia card is probably better at being an efficient performer, though - less power consumption. But, the Nitro+ is probably the best card in that series..... the RX 580 Nitro+ is probably shorter than most 1070s too so will be easier to fit in most cases? That's just a minor observation and might be redundant depending on what case I choose.
I don't know which form factor yet so please, unless there is another reason to include all that - only compare to the two cards and the other factors?
If I have to think about case sizes and power/heat issues.... ugh, too complicated. :)