I think both choices are valid options in that kind of situation.
But I choose also to help Lily. Why?
#1 Because as some here already stated, Larry wasn't dead/zombified yet, so there could have been a chance to still safe him.. and as tiny as this chance may be, I at least would always help and try safe him.
#2 Just as Rick Grimes in the TV-Series said to Daryl: "We don't kill the Living." And Larry, not yet zombified, does count for me still as a human I could save through CPR. As simple as that.
And I give a rats ass about all these ifs and whats in regards of Larry beeing revived as a walker and the problems you (Lee), Kenny, Clementine and Lily would have to face. Because, the game/story isn't programmed to go that way. If it actually were like that, you try helping Larry, Kenny doesn't throw the saltblock on his face, Larry revives as a walker and bites someone else in the aftermath... TOTALLY DIFFERENT STORY. I would probably instantly play that chapter again and choose otherwise.
But, as amusing as this discussion is, what bothered me the most was the fact that Clementine had to see all of this. With her reaction afterwards I was so furious I seriously wanted to slap Kenny around a bit (so kudos to the Devs for making the game so emotional).