So anyhow, I didn't mean to give you such a short answer earlier. The GTX 1070 is a good choice because it's about equal to the 980 ti in terms of performance, but is itself about 1-2 years newer than the 980 ti depending on when it was originally purchased, assuming you'd be buying it used.
If we're being honest, I don't like the idea of buying used cards because often they are not that much cheaper than an equivalent performing newer card and it's always like rolling the dice when you buy one. Yes, some people are lucky enough to get a good deal and get a card that lasts a while. Others, pay 75-80% of what the card cost new and it lasts a few months if that. Often they have problems right from the start which is why they were being sold in the first place. Ebay cards are notorious for this, no matter what anybody has to say about it.
If you could find a GTX 1660, 1660 ti, 1660 Super or RTX 2060, or an RX 5600, at a price you could afford, new, you would be MUCH better off, because you'd not only get a full warranty, you'd get a card that has a full lifetime of use ahead of it, instead of already having burned through at least half of it's useful life. And if you are buying used and can't get the card for less than 50% of what it cost originally, I wouldn't even bother with it. Save your money until you can afford a new part. Even in Argentina there is hardware to be had, new, if you are willing to bide your time or pay a little extra to get it brought in. Usually it is worth it.
If that's not possible, then just be very particular about what you buy used.