the ASICminer usb's are hashing around 300 with a power draw of ~500mA (if I am correct) thats about .5 watts.
You are right about Erupters being a better deal.
BTW, they draw 500mA which is about
2.5 Watts. USB runs at 5V. Just saying...
That means you could run 100 of these Usb ASICs to pull the same power and ONE 7970.
The Erupters do about 330mh/s so 100 of them would do 33.3gh/s at 250w
7970 does ~700mh/s for ~200-250w.
Doing the math the Erupters are 57.57x more efficient, power wise, than a 7970.
Erupters being around 2btc which is about $260 at today's exchange rate.
7970s being about $400
100 Erupters would be $26,000
Enough 7970s to reach 33.3gh/s is ~48 cards.
48 7970s would be $19,200 but then you have to figure you have to have a power supply and mobo/cpu/ram for every ~6 cards. Thats $22,000 for 33.3gh/s in gpu miners and they will be pulling 12,000w vs 250w for same speed using Erupters.
So final numbers.33.3gh/s miner using Erupters would cost about $26k at a power consumption of 250w
33.3gh/s miner using 7970s would cost $22k at a power consumption of 12,000w
Final conclusion.
In the short term gpus are the best deal but once you figure in electrical cost, heat production, and space savings the Erupters win hands down.
So one miner would use the same power as a couple light bulbs and the other miner would use about the same power as 3 or 4 houses.