I think skill-wise both are about as tough of a challenge. HOWEVER, I feel that stylistically Bendo is a much tougher match because he has an extremely solid ground game. Showtime is about as good as they get striking-wise and I feel that Aldo would keep the fight standing, which would give him a better chance.
I think that both matchups are extremely tough, but based on recent fights, how they match up stylistically, and size difference, I'd say Pettis has a better chance of beating Aldo (by a very slim margin tho)
If I had to put odds on it, I'd say Pettis has a 35%-40% chance of beating Aldo for a title shot and about a 30% shot at Bendo (primarily because I think he'd get outworked on the ground)