Go for this
http://www.lacie.com/asia/products/product.htm?id=10621
future proof too
TB will be expensive while USB3 will be a lot more affordable.
actually for normal usage you wont notice much for either setup you go for.
For OS & application usage 256gb will be more then enough.
You can go for USB3 for the start and try it out and if the speed is not to your satisfaction.
You can look around for TB SSD.
Lastly, remember not all TB casing give you high speed as there are quite a lot of existing TB casing not using SATA3 or 6G chip.