They are really not comparable. The arduino's CPU is a microcontroller, its dumb, slow, but has a lot of IO. The PI's CPU is an ARM, its a real computer, it can do things like decode HDMI video on the fly. But it doesn't have many IO ports.
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!