I used to play in some underground 50nl - 200nl live cash games, but I spent way more time as a dealer in those (it was more profitable and my family owned one of the rooms). I played mainly to start the tables when there weren't enough players.
This is my lifetime graph for playing in live cash games:
Overall you'd have to play at least 200nl to do it for a living (depends on how much per hour you'd like to make).
Live poker is definitely way softer, so your win rate can be a lot higher, however, the number of hands per hour you play is way lower. Online you can easily play 1000+ hands/hr, while live you can be happy with 30-50 hands/hr.
Therefore, even if you're crushing 200nl for 20bb/100 (after rake and tips), you're still only making 12-20€ per hour. You have to also consider the time that it takes you to drive to the casino and the time you spend waiting for the game to start.
The variance in live games is also a lot higher, because of the low amount of hands you play per hour. You need a bigger bankroll to have the same hourly as online (but it's easier to achieve this hourly).
Overall, live poker is easier, more fun and you meet many interesting people. For playing poker seriously (professionally) though, I obviously chose the online option as it gives me more constant income and an easier way of tracking my play history/improve. It also challenges me more.