While I agree that kung fu Panda would whoop both of them, twice, I will stick to the question at hand.
hmmmm let's see... Jean Claude with his Karate (Taekwondo, Muay Thai, Shotokan Karate) 18 - 1 competition record, fancy telegraphed kicks and impressive flexibility.
VS
Seagal, aikido master (not many of them), 7th degree black belt in karate, and belts in judo, and kendo.
I'm going with Seagal.
Van Damme's form always comes off as hollywood. Splits, lots of telegraphed high spinning kicks... it amazes me he still has a groin. I don't think Van Damme has dedicated himself to martial arts in the way Seagal has. That and I believe that an aikido master can take down a taekwondo black belt.
+1
i lived in so cal in the eighty's when both were making their mark in the martial arts/movie community.
i spent a lot of time with the folks at black belt, and inside kung fu magazine working with photos for my sifu and related articles.
it was pretty much opinion across the board that Van Damme was a poser and that Seagal was the real deal.
there was a huge dust up over JC's lack of control causing the partial blindness of one of his co-stars in a movie
from people magazine~
"Actor Jackson Pinckney was awarded $487,000 by a North Carolina appeals court on Sept. 6 to compensate him for being partially blinded by Jean-Claude Van Damme, 34, during a stunt for the 1988 film Cyborg. The judge found that Van Damme engaged in "excessive contact" by stabbing Pinckney in the left eye with a rubber prop knife during filming. As a result of the injury, Pinckney, a Fort Bragg-based soldier, was medically discharged from the Army. "
do a cursory search on Seagal and you will find that his opening of an Aikido school in Japan was a first by a non-Japanese.
that my friends is impressive enough on it's own.
http://en.wikipedia.org/wiki/Steven_Seagalas a student/teacher of Chinese internal martial arts for over 20 years my opinion is biased because Aikido and the Chinese internal styles are related, but I've got respect for them both.
on another note, SS released several Blues Albums and IMO they aren't bad .