If you're a defense lawyer you can always choose who to defend, and if you're mandated by the state to represent someone you don't have to try your hardest.
Either way, if you're smart enough, you can pick your battles wisely, even with murderers.
Sometimes they'll just admit to their lawyers, like in the Tim McVeigh case.
I'd definitely go with defense, for the same reasons Flame posted, and because defense lawyers have a greater potential to make the prosecutor look stupid than the other way around.