As seen in these pics the spade bit has a very long spur. The Fostner bit on the other hand it has a very small spur. Also the Fostner bit has a cutting edge on most part the cylinder . So it will be slower in the wood as contact area is more but cut will be smoother also because of short spur you won't get a very long divot at the circle being cut. This especially useful in making holes in thin stock where there is not much wood left at bottom of hole.
The spade bit on other hand will cut faster and will give a rougher cut. So it's suitable for through cuts.
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