So a TB HD would be no faster then a USB?
You mean USB 3.0?
Potentially yes. a 5400rpm and 7200rpm platter HDD drive in a usb 3.0 enclosure will likely operate at the same as TB. As for SSD's there are some very fast SSDs out there and some that are not much faster than a platter HDD, but when you are talking about SSDs and potentially running them in RAID 0 then a TB will provided potential benefits over USB 3.0 as it is potentially twice as fast.
I have a RAID 0 array of two 4TB 7200 HDD drives in an enclosure that has usb 3.0 (5000) and sata II (3000). It operates at about 215 megabits per second in read and write and operates the same speed when i plug it into a lacie TB esata hub as well. So the drives are the limiting factor, not USB 3 or esata or TB.
The TB 'drive' consists of an enclosure with a controller (little circuit board with a sata connector and the hard drive its self. the speed of the hard drive (SSD or HDD (7200rpm or 5400rpm) is what determines the speed the TB protocal operates at because it uses so minimal bandwidth available in TB. The hard drive itself will not even come close to saturating the available TB bandwidth so for this reason the hard drive is the limiter in a TB enclosure.
There is no 'TB hard drive' per se. The hard drive that is used is still the same as the one you would put in your mac (albeit many options there).
Have a look at speeds on the web for different connections. for example USB2 is 480, firewire 800 is 800, sata II is 3000, usb3 is 5000, sata III is 6000 and TB is 10,000. The current mac sata connectors operate at sata III while TB operates at 10000.
You will never go wrong with a TB enclosure (to future proof your self as hard drive speeds increase) as they are the fastest. the limiter as I said is what drive you put inside it.
They are also super expensive so a USB 3.0 enclosure will almost always give you similar speeds until SSDs get faster (unless you are running RAID etc)