I think there are a lot of factors to consider here....
playing live you are more attentive to the game, there is less to distract you so you can focus more on the action even when you are not involved in the current hand. Also, you are seeing less hands per hour so you are less likely to feel the itch to get involved. You get to know your opponents more: their mannerisms, body language, personality etc. Also, you probably made an effort to get to the casino so you take it more seriously.
Online... it is easy to get distracted and much harder to follow hands you're not involved in. The action is thick and fast and you will feel that you are folding so often because the action comes around so quickly. I also give less credit to players hands online than live... this is clearly a mistake and something I am working on. I sometimes get a paranoia that people notice me raising often and when they come over the top I interpret this as a
bluff only to find myself up against pocket Kings... I think the remoteness of online play contributes a lot to this, because we can't see who we're up against and we are much more afraid of folding to a bluff.
Also, playing online is affected by the time of day that you chose to play. If you are playing on a site that accepts US players at 2pm EST during the week... you are more likely to find people who play the game seriously. Log on at 11pm EST on a Friday night and you will get much more drunk recreational players!!!
One thing is true though... if you can beat say 2/5 online, you can crush live limits much higher than that. Something I am working on all the time lol.
The beauty of online though... you can get up and leave anytime you want without feeling guilty for taking everyones money and running!!