In my place, the 970 and 390 cost the same. So for me, it is better to buy the 390. It also has 8GB memory.
If the Ethereum last more than 2 years, we might need big memory size to minimize the hash rate decrease.
That is right. That is the reason why I mine with 8 GB and 4 GB cards. I have 280x, but they are hashing less and less.
Ethereum will not last 2 years. I am pretty sure that it will go POS before the 2GB cards can't mine anymore. Buying GPUs with 4 or 8GB is a waste of money.
Even if it doesn't go POS, then the difficulty will be too high for any decent profits.
It's due to go POS sometime next year. Looks a bit marginal for anythign past ONE year at this point, and the DAG size growth is guarenteed to keep eroding hashrate.
X11 ASIC have been seen - but in TINY quantities that appear to have had no noticeable difference on the network hashrate. That SHOULD change soon, but it's not happened YET.
Only 2Gig cards I have (and am mining with) were either already on hand (my NVidia stuff and some leftovers from Litecoin/X11 mining days), one VERY cheap HD7870 that's well on the way to paying for itself, and a couple of GTX 750ti small form factor cards I was going to get for the machines they are going into anyway eventually.
I doubt I'll bother with any more 2Gig cards - I've pretty much filled up every open PCI-E slot I had available (living situation RIGHT NOW precludes going open-case and risers, that should change after I move this summer) so even cheap used stuff is about out of attractiveness for now.