Maybe it's just the hot room I'm sitting in right now, but man, the thought of gaming with a pair of 390s sitting in the case next to me, heating everything up is just not appealing to me in the least.
It's like with just about every GPU purchase, you lay out your unique situation and then pick the option that matches that special circumstance. Of course a pair of 390s in Crossfire is mostly/usually going to be faster, but that doesn't make them always better.
What resolution are you running? What CPU? What power supply?