I'm going to come out and say it, "Planes" is better than "Due Date." "Due Date" tries to be a bigger and better version of "Planes" and fails, so instead of having the flight land in a different city there's a ridiculous bomb threat. Instead of one major story point we have multiple plot details that didn't have to be written. These things and more amount to an ending that should've been reached through a faster and easier route.
The characters are exaggerated to the point where Ethan (Zack Galifianakis) is just the most grating idiot anyone can meet and Peter (Robert Downey Jr.) is a ticking time bomb. They both have their redeeming qualities, but you don't always see them shine through. Instead Peter physically and verbally assaults Ethan, and Ethan seems unintentionally hellbent on ruining Peter's trip. While you will laugh at the movie a lot you'll notice something is missing, even if you've never seen "Planes." If you haven't figured it out yet, I'm talking about the heart of the movie.
"Planes" has several scenes that won't make you laugh, but will leave you thinking something like,
"What was this movie really about?" "Due Date" doesn't really have that, you go in, have a laugh (a pretty good laugh at times) and get out.
Both movies are great, one just does the genre (road movie) justice. This is what it boils down to, "Planes" did it first and did it best. Does that make "Due Date" terrible? No, it's a great movie. It's just in second place.