Is it healthier to have an apple with PB or string cheese?

I have to gain weight for medical reasons (but don't really want to gain) so of course I would rather have the cheese because it's already portioned for me. With the PB, though, I may put on less than what I should. Which, though, would be healthier? I've been eating cheese lately (pizza, burritos, nachos, etc.--I'm not usually so unhealthy), so I was thinking I should take a break from that and have the PB or does that really matter?