Are you are planning on buying an external hard drive and you want the quickest backup possible?.
According to recent MacWorld tests if you are buying a normal spinning hard drive then the speed differerence between USB 3.0 and Thunderbolt is not much and not worth the extra cost of Thunderblot. Both Thunderbolt and USB 3.0 were roughly 3 times quicker in real world testing thann USB 2.0 using spinning drives. If you are buying an SSD there is a bit better than 5% speed benefit for Thunderbolt over USB 3.0 unless you are using very high end RAID drives where the speed difference is about a factor of 4. In real world testing SSD drives were just under twice as fast as spinning drives for both USB 3.0 and Thunderbolt so that equates to 6-7 times faster than USB 2.0.
So:
If your computer has USB 3.0 best value at this stage is USB 3.0 spinning drive (x3 speed of USB 2.0).
If you want the fastest best value then USB 3.0 SDD (x2 speed of USB 3.0 spinning).
If you want the outright fastest then Thunderbolt SSD (x2 + 5% of USB 3.0 spinning).
My purchase: LaCie 1TB spinning drive with USB 3.0 and Thunderbolt connectors as I don't have USB 3.0 on my current iMac.