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.
According to my math, depending on what you pay for electricity it would take 8 months before the USB miners start to overtake the $4,000 in additional investment due to power savings. After that the USB miners are ~ $450/month cheaper to run. Also, according to my math you can do a 2.4 Gh/s rig for ~$1,500 using 7950's. That's $21,000 for 33.3 Gh/s, which puts it out to 10 months for the USB sticks to overtake the GPU's.
Even so it seems to me like the USB miners may be a better price than I initially thought.