my experience has been that SD cards (even the slow ones) are faster than flash drives for the money. When you can get a Class 6 SD card for less than $20 and to get the same speed in a usb flash drive will probably cost more than $50 as I recall.

I'd have to research the specs, but anecdotally, my $20 4 gb USB flash drives seem beastly slow compared to my 8 gb C6 SD Cards. It would require a stop watch to compare the difference between my newest USB Flash drive and the C2 sd cards I just recently bought.