Probably online shows a broader knowledge, live shows quicker calculation and pattern recognition. Experience will probably be a bigger factor in live because some things you will just know rather than having to calculate.
Live is probably a better judge, seeing as both players have the same amount of time, whereas online some people have more time to devote to each move. I'm better the slower the game though, my 30 min play is much better than my 15.
It seems that everyone's online chess ratings are about 100-200 points higher than their live chess ratings.
I know that its difficult to compare these two, but which type of chess do you think is a better indication of one's chess skill?