Since you already have a 760, I would recommend SLI. There's no way you would get anywhere near the same performance with a HD 9000 series card for the same price ($250). Even in the best case you'd have to pay more than $600 to get anywhere near the same performance as the 760 SLI would give you.
That said, I'd still advice waiting unless you have some game that doesn't run adequately on your single 760 right now. If you wait til HD 9000 hits the stores, you are likely to get the second 760 somewhat cheaper. If you can wait til Maxwell comes out, the second 760 may cost considerably less than it does now.
However, if your GPU is struggling with a game you play a lot now, then it's worth upgrading right away if you can afford it.