It is always better to buy latest and updated versions for GPUs as they are more efficient and sometimes cheaper or slightly expensive than the older/previous models.The grey/used market is also a good source to buy GPUs and much discounted price especially when the market is down as newbies tend to sell away to cover the losses and exit mining
i personally think that anyone who buys a 20-series card for mining is going to get burned.
they are outrageously expensive, and it looks like they will use even more power (judging based on the HUUUGE HSFs on most models). I really doubt they will be much, if any, more power efficient than 10 series cards.
i'd love to be wrong, but i just dont see it.
for example, a 1080ti is the most power efficient card on ETH with the ETHpill.
50-55 MH/s @ about 150-160W. and they cost under $500 right now.
if you assume the 20 series cards will be the same power efficiency, this is what kind of hashrates they'd have to produce to justify the price over a used 1080ti. (based on EVGA prices for the cheapest 20 series models they have right now)
2080 - 80MH/s
2080ti - 120MH/s
i just dont see that happening.