The benefit of the usb enclosure is you have somewhere to put your original drive when you take it out of your computer ...? what kind of Mac do you have? If you have an intel Macbook or Macbook Pro you can boot from a usb external enclosure...
Bottom line is: it's cheaper to do it yourself, and you can get enclosures for under $15. Just make sure it's for SATA 2.5" hard drives.
You can use SuperDuper or Carbon Copy Cloner. Attach the new drive to your laptop via a usb enclosure or usb-to-SATA cable. Run Disk Utility, locate the drive, and "erase" it, using MacOS extended journaled format. Then, run Carbon Copy Cloner, or SuperDuper, and choose your original drive as the source, and the new drive as the target, and clone (make a bootable copy.) When it's done (and it could take a couple of hours depending on how many gigs have to transfer) then you can swap the drives, and you're good to go.