Main line or offbeat?

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. 

I think the lower-rated the opponent, the greater the impact of surprise.

So for opponents in the 1600-2000 range, some 'forgotten' sidelines may be very potent weapons.

It depends on your time available, your memory and your taste.  If at all possible ML will give you the better game.  With an off beat opening you are hoping that your opponent will go wrong and make a mistake in the early part of the game.  "Hope" chess is usually not the best game to play.  On the other hand you can plan your openings to limit your opponents options and how much you want to learn.

For example, you have black, white plays 1.  e4  you have at least 6 good options but most leave a lot open still for white.  1. e4 e5 can get you a Ruy, Guiocco, Scotch, Kings Gambit, Vienna, etc.  but if you play 1. e4  e6 you are almost certain to get a French of some flavor, good if you like French.  If you don't like French there are other options.  Play with the game explorer and see what you get.

If one doesn't have A) a very good memory and B) a lot of time patience and discipline and a rating of, I dunno, +2000 -- I think it is very difficult to have anything like a true opening repertoire. What one has is, a little of this a little of that, a trap here, a pet line there, a good grasp of what happens in this corner of this opening but my god don't play 7.f3 cuz I have no idea what to do then, etc.

I advocate studying taking an opening one wants to learn and looking at many master games using that opening taking careful note of the transition to the middlegame/where the book ends and seeing how play develops from there. But is this really an efficient system? I don't know. I enjoy playing over good games -- it's its own reward.

It depends I usually try to avoid mainlines but some openings you can't. For example I've been trying offbeat lines of the Ruy as black for awhile, and higher rated player that understand the opening crush me. I finally gave in and started play the marshall attack and  I've had pretty good results. Here's a list of offbeat openings that work for me.

As white:

Giuoco Piano Bayonet Attack

French Defense Winawer Variation 4.Nge2

As Black:

King Gambit Falkbeer Counter Gambit

Two Knights Defense Traxler counter attack

Budapest Defense Fajarowicz Variation

you should know the main line, but most moves not in the main line but in book are completely sound, so you should pick the line that's best for your style.