if i was mining BTC, I wouldn't buy anything. if you insist on buying something though, you should buy 7790's or 7950's. there have been 7950's for under $200 in the last 2 weeks or so. i'm thinking they're a better buy than the 7790's now.
why? because the USB crap will lose its value fast, and it's good for nothing but bitcoin mining. while your video cards will basically mine two things (jack and shit), at least you can resell them for a good percentage of their original purchase price
Correction, USB miners can mine anything with SHA-256 algorithm. So, it`s not just for mining bitcoins, but also for alts. I say mine alts and just sit on em for a bit.