If I were you, I'd put the SSD in the laptop and use the original mechanical laptop hard drive in an external USB 3 enclosure.
Putting an SSD on a USB bus is kinda hamstringing it, I think. USB3 is fast but I think the SATA2/3 bus would be faster. Some testing would have to confirm this, though.
If you must use an SSD externally, I'd be looking into E-SATA, assuming your laptop has the port.