That depends on what your preference. The Droid Pro is similar to the BlackBerry world, but it has a lower resolution screen and is on Froyo 2.2 (GB 2.3.3 rumored for later on summer or longer). The D3 has higher res display, full keyboard that a man can type on.
While this analogy will get me torched, one is a BMW 3 series and the other is a BMW M3 series, same mfg, just one has a bit more to it than the other.