I will comment first about SLI/XFire.
SLI and XFire is only good if you have already a card and want to add another one.
If do not have any card, a more powerful card is always the better solution.
Multi GPU solution is not always supported by games and could have driver problems (micro stutters, etc.).
Note: I have never seen any driver issues on 2xGTX970s on SLI so far, but based on experience, the risk is always there.
About GTX980, this is not really a good buy. You are paying quite a lot more for something like GTX970 with 0.5GB more RAM. Aside from the extra RAM, it is only 5% faster than GTX970.
GTX980 TI is a good choice. From nVidia, only GTX970 and GTX980Ti have good price/performance value.