The other big thing you have to consider is resale value. You can plug numbers into ROI calculators all day, but bitcoin mining is so unpredictable that you never know what is going to happen. Always having a viable out with hardware value to offset you expenses is a huge bonus. I'm currently sitting on probably $15k in hardware at resale value. It'll be relatively easy to move as it has a diverse market. Selling 100 usb powered ASICs in an uncertain market, to a very small niche, 6 months from now, is an abolute crapshoot.