Which do you think is the most practical approach:

 Booking up mainlines. When the opponent deviates it's typically not going to be a prepared line, so you have all the advantages inherent in a main line?

Booking up and diverting the game to the lesser known lines, even if they are not the current ideal, and usually slightly inferior to the main line. In effect a prepared line but not one with the teeth of a pet line.

For opponents 1600-2000 let's say. 

It depends on your taste really. You shouldn't expect "advantages" from the main line if your opponent leaves the book line  because you're not guaranteed anything. He may be trying to leave the book and force you to "play chess." But whats most important is that you understand the theory. That way if you leave the book, you still have concrete ideas in mind.