RE the post above:
[[ I would get a usb 3 external with UASP op. Even with trim, I only got 380 read/340 write on a crucial m500 480 gb boot drive. With usb uasp, I get 440 read/400 write. ]]
As another reference point, I use a Crucial m500 240gb as an external boot drive, mounted in a plugable.com "lay-flat" USB3/SATA dock with UASP support.
Read speeds are 432mbps and writes are 276mbps.
I believe the lower write speed is due to the size of my drive, which is half that of the poster above...
As pointed out, the disadvantages of booting via USB3 vs. thunderbolt are that you lose the "TRIM" feature (although some newer SSD's don't seem to need it), and that firmware updates may become problematical.
It depends on whether these are "can't live without" features for you.
In my case, I'm willing to live without them...