It's recommended when you want to install an OS to first and foremost consider the internal SSD for faster performance and response time. If you allready have an internal OS, and don't want to replace it, or simply want to install an OS on an external drive, it is recomended to use performance USB flash memory drives (SLC drives over MLC), since they will boot the OS faster than MLC Flash drives or SD cards.
I've done some tests on good flash drives, and have some suggestions to make for drives with low latency, and when an OS installed,have very low boot times:
Prices are for reference, recorded on sept. 14th 2008.