First off...you be better off taking your mining funds and just buying the cryptocurrency of your choice.
If you are attaching mining devices to a Pi and are just concerned about power use by the Pi, an A+ is the lowest power use (and least expensive) Pi there is. If you need Ethernet and/or multiple USB ports, then the B+ would be the cheapest to operate. If you are going to mine directly on the Pi (and, therefore, need all the processor power you can get...from a Pi), then you want the Pi2B.
But in the end, the odds are that no matter how you set things up, you're going to spend more on power than you will get back in cryptocurrency.