I think it's dependent on the trainee. I don't have a problem doing chins with 2 plates, but generally only do 70lbs for pull ups.
The weird thing is, I'm weaker with a neutral grip. It would make sense for my neutral numbers to be in between, but they are not. It's even weirder when you think that most people are strongest (from what I've seen) with a neutral grip.
Generally, if you have a stronger back than biceps (relative), you'll be better at chins. With the opposite, you'll be better at pull ups.