Concur, Go for a seperate enclosure and for the HDD: For 3 1/2 in dives go with a WD 1TB Black or the Samsung F3 1 TB sata II drive. For 2 1/2 In Laptop drives go with 7200 RPM UNDER 1 TB.
Real life performance on USB3 can be a mixed bag:
.. The IB (Z77 MB) are the only MBs that offer native support for USB 3, previous CPUs/MBs use a 3rd party controller. Make sure the controller has the Latest firmware, and driver.
.. Real life performance will also very widely based on the files read/written, ie Sequencial large file transfers may be good, But doing a large write small file transfer - may me subpar.