personally, what i check is the price/performance ratio, like if you use the 970 as base line. how much faster is the 980 vs the 970, then how much more expensive the 980 is over the 970. if it's, say 15% faster but 30% more expensive then no.
also consider the temperature, noise, etc.
but if money (among others) is not an issue, then by all means, you can also setup sli if you wish.
most of the time, i don't buy the high end, at most i buy the 2nd in rank, mostly third (560Ti, etc)