I have been playing Poker both Online and in Live Tournaments for a while now and while I tend to fade in and out of both at times I have found that the different formats complement each other fairly well. The underlying game of Poker is the same, but there are a number of significant differences between playing Online and Live that take some getting used to.
For Live Poker there can be a lot more focus on Playing the Player and being able to read body language to pick up Live Tells is an important skill. As is being able to Banter effectively, not just to keep the game fun and engaging but you can pick up big Tells from table banter too. You also have to be very aware of the tells you might be giving off yourself so you really have to have your Game Face on at important times and need to be on the lookout for players Angle Shooting. Maintaining focus and discipline can be quite exhausting.
For Online Poker there are still tells and you need to play the player too, but you really can only profile them by watching their Betting Action and the hands that they play so it is a lot harder. Online the Chip Management is taken care of and you can get a lot of help with calculating relative chip sizes, pot sizes and the tempo of the games is a lot faster. You often get moved from table to table quickly and don't have to worry about masking your reactions, or other peoples Table Etiquette so in many ways it's a more relaxing and non-confrontational way to play.
Personally I tend to find Online Poker to be a bit boring after a while because I usually feed off the Table Banter and I like the social aspects of poker. However I also find that if I don't play online regularly, my Live game performance can slide. Playing online is great practice for certain parts of the game and it's really important for me to test different strategies and situations that I might be working on. Since online poker is faster it also means I can play a lot more hands so its a more efficient format to hash things out in.
After a recent run of OUTS in my Live Poker I am trying to get in some game time playing Online to recalibrate my strategies and reset my mindset. So far so good...