There has been an argument over whether break cues should be light or heavy for years.  My definition of light cues is 18 oz. or less.  My definition of heavy cues is 20 oz. or more.  I base this on an average cue weight of 19 oz.  Having done some reading on the subject, I came to the conclusion that it comes down to personal preference.  However, that wasn't good enough for me, so I did a little more research, and came up with some physics arguments which seem to favor lighter break cues.

Break cue weight preference should be based on simple physics. Here's the "simple" physics:

The momentum of the cue ball is what matters, relative to the rack. The equation for Momentum is Mass X Velocity. The mass here isn't the mass of the stick, it's the mass of the CB. That mass is always around 6 oz, it isn't going to change. Therefore, velocity is the only factor we need to take into account.  All we have to worry about is propelling the CB as fast as possible (while still maintaining control, of course). This is known as Impulse. Impulse is Force X Time. Force is Mass X Acceleration. Time is contact time between cue stick and cue ball. This mass is the mass of the cue stick, acceleration is the change in velocity of the cue stick from rest to contact.

Summary of equations:
Momentum of CB = Mass of CB X Velocity of CB
Impulse of CB = Force generated by cue stick X Contact Time
Force generated by cue stick = Mass of cue stick X Acceleration of cue stick

Intuitively, it looks as though, since mass (of the stick) and acceleration have an equal effect on Force, that a higher mass cue will result in more Force, which in turn creates a higher Impulse, which in turn creates more Momentum for the cue ball. However, look closer. A lighter cue will allow the acceleration component to increase. For instance, let's say we have a 21 oz break cue. Let's say we can generate 20 mph with that cue. That creates a force of 420. Let's lighten that cue by 3 oz to 18 oz. Now we might be able to generate a little more speed, let's say up to 25 mph. Now the force is 450. Obviously, there is a point of diminishing returns at either end of the mass spectrum (I used weight of the cue instead of mass for these calculations, but you should get the gist). Extremely heavy cues CAN increase force, but only if acceleration isn't diminished to a point where it hurts the amount of force being applied to the CB. So, the current trend in thinking is that lighter cues are better because they allow higher velocity, increasing the acceleration slightly more than the decrease in mass, which generates more force. As is the case with everything else in pool, you'll have to find the cue weight which allows the highest force applied FOR YOU. It isn't the same for everybody.

I prefer a lighter cue because it seems easier to generate velocity. It seems to take more effort to generate velocity with the heavier cues for me. However, I've never done any empirical tests on this. I know Bob Jewett and Robert Byrne have, though, and they found that lighter cues up to a certain point do increase the Force applied to the CB.