It's also important to remember that both of these are really "product ranges" rather than just single products. Admittedly the range of Raspberry Pis is currently a lot smaller than the range of Arduinos, but there are still
three major versions "in the wild". And there are also
moves toward an ecosystem of third-party "Raspberry Pi compatibles"
There are so many different types of "arduino" that it's almost impossible to keep track. From the thumbnail-sized
DigiStump Digispark to the 32-bit ARM-based Arduino Due and boards with every kind of built-in extra.
And just to add to the fun, don't forget other popular entries in this crowded space such as the Beaglebone Black, Cubieboard, Freescale Freedom boards, and evaluation boards from just about every chip manufacturer!