Hard choice but I'd have to go with ninja. The only real advantage the shinigami have over the ninja is that they're already dead. So most ninja couldn't kill them. But ninja are sooo much trickier.
Ninja fight at a distance when they can and only fight close quarters when necessary. Plus they always keep themselves concealed (provided the enviroment alows it). And they're agile. Very, very agile.
Shinigamis.....well......they just stand out in the open and swing fancy swords all day. But they do no good if they can't hit their target (only exception I've found is Aizen's hypnosis zanpaktou). Plus, most shinigami seem to require a zanpaktou's help to become the least bit agile. Quick they may be, but agile, I don't think so ;) .
However in the end I will say I'd much prefer something more like Yoruichi's skill set. A little bit of both :)
