As I said it depends on how much you can get your hands on certain cards. To me for example a 980 Ti rig (or even just a 980 non-Ti rig) would be suicidal. And then again putting 6 of those cards into one rig would require a very pricey PSU unless you don't care about RMA time or resell value, or don't mind using two PSUs which I personally refuse to do.
In my personal case 960s are sligthly more efficient than 970s and 750 Ti's but I decided to only use 2 types of cards in order to simplify things so I'm only buying 970s and 750 Ti's until Pascal comes along which allows me to know the efficiency of the cards depending on the algo without having to calculate it. More cards would just complicate things.
Using 2 PSUs with USB powered risers +
http://www.aliexpress.com/item/Add2PSU-Multiple-Power-Supply-Adapter/1743678544.html?spm=2114.01020208.3.2.oKvSMv&ws_ab_test=searchweb201556_2_79_78_77_80,searchweb201644_5,searchweb201560_9 is very simple and secure in my eyes.
I've been using this setup for something like half year.