As James said, I'm sure many will get the wrong idea about ninjas. From what I've gathered about them, they were nothing more than spies who used covert weapons (shurikens, small swords, etc) only when their life or mission was at stake. Someone please correct me if I'm wrong.
I would be a samurai. Why? Numerous reasons. As samurai evovled, they eventually got this sense that "Run away and live to fight another day" was dishonorable,which is why I love their bold spirit and willingness to die for their lord, for an idea. Not to mention that samurais were originally called Warriors of Horse and Bow (I forget where I heard that) so they'd undoubtedly be excellent horsemen and marksmen. And then bring in swords. Samurai were probably the most accomplished swordsmen ever and used what is, in my opinion, the greatest swords designed ever. Katana's were possibly the most versitile weapon and had all of the strengths and almost none of the weaknesses of other swords (ie: sharp, fast, light, durable, small, etc). Taking into account that samurai trained in Iaido (the practice of respecting a sword, counterattacking from any position at any time, sheathing and unsheathing quickly and efficiently, and more) as well as Kendo (basic sword fighting techniques). All of their skills, ideals, and general outlook on life is truly amazing. I'd become a samurai over a ninja any day.
Restore the Ming Dynasty in Beijing!