"A whole hell of a lot" means "very very much." In other words, "a whole hell" emphasizes "a lot" to strengthen its meaning. The passage just means that "it is *so much* better to be the defense attorney of someone who you believe to be guilty than to be the defense attorney of someone who you believe to be innocent." (I guess the idea is that if you "lose," you don't mind as much because the person who is guilty should be punished anyway. However, if you "lose" while defending someone innocent, as a good person you might feel very bad about that.) To keep the sentences in the same tense, I would say that the words dropped are "It is" - so you're definitely got the right sense!