We need a graphics driver either from Apple or Nvidia. It's far more likely that Nvidia will update their WebDrivers at some point, as Apple has absolutely no interest in supporting us. In fact they've even removed Maxwell support from their drivers, although it's based on a revision which should support is.
It's hard to predict if Nvidia is going to support us (again). In the past they've been quite supportive, although it's not unusual that a new GPU remains unsupported for some time. No one outside Nvidia or Apple can now that for sure.
Personally I'm quite sure that Pascal will get supported at some point, since virtually all Nvidia GPUs received support sooner or later. Hopefully we don't have to wait for the next major release of OS X next summer (big OS releases are usually accompanied by graphics driver updates).
Besides that it's not a new situation to me. As a long-term hackintosh user I'm used to the fact that I can't go into a PC store, pick
any part and expect it to be compatible. We've been in a very lucky situation lately, since this was possible with lots of AMD and Nvidia GPUs in the last years. With Apple focussing more and more on pink laptops and fancy Emoji Bars, I'm afraid that we'll lose support for high end hardware. We'll see, time will tell.
To answer your question: I'd get the 980Ti, it's still a awesome card.