Some of the higher class (i.e. expensive) SD cards are pretty fast. My Class 4 cards are about the same speed as my USB 2.0 sticks; I've seen cards as high as Class 10. I don't have USB 3.0 (yet, 'tis coming soon) to compare to but, considering USB 3.0 can be as much as ten times faster than USB 2.0, whs is probably correct about USB flash drives being faster on USB 3.0 than cards (he's usually correct anyway).
Usually, one uses cards in cameras, vid cams, phones, tablets, etc. that keep the device semi-permanently installed within the device whereas USB flash drives will stick out of the device and is intended to be used temporarily. A card, even when inside its little storage case, takes up less room than a USB stick but that also makes it easier to loose the thing. Every computer has USB ports but not every computer has a built in card reader. Portable card readers are available but are just somethiong else to lug around and keep track of.
Both cards and sticks have their advantages and disadvantages, depending on their usage. I have four Class 4 8GB SD cards I use in my two cameras (two are spares). I also have a Class 4 32 GB card I regret buying since it's safer to have several smaller cards instead of one to minimize data loss should one fail. I keep it in my purse, along with the two spare 8 GB cards I have for my cameras, to use as a back up drive for photos when on the road (all my portable computers have had built in card readers). I also have a lower class (not sure what class it is since it uses a proprietary classification) 1 GBSD card I use on my e-book reader). For just about everything else, USB sticks are more convenient and easier to keep track of.