Note: as of July 1 2009, pheonlic tips (and other really hard tips) are now illegal in BCAPL. It s likely that these will become illegal in APA over the comming year. These hard tips will leave little divots in the cue ball when struck hard (BCAPL got tired of replacing cue balls all the time).
The weight of the break cue should be as LIGHT as you can accurately play. A light cue accelerates faster and hits the cue ball faster and ultimately makes the cue ball hit harder. As long as you don't go under 18 oz you have little to worry about in the "its too light" catagory.
A shaft for a break cue should have a straight taper--this gives it added stiffness and adds power to the break.
The tip for a break cue shold be in the 13mm-14mm range and should be closer to a nickle (or even a quarter) than a dime in radius. In addition, you should get the hardest tip you can find (like Talisman XX hard with a durrometer reading near 98-99; but realistically anything over 90 is fine for 95% of us) so that all the energy goes into the cue ball.
There is no advantage to a brass ferrule compared to a lighter plastic ferrule in a break cue. Brass ferrules are more for Massé cues where nose weight is advantageous. With a super hard tip, and the power being transmitted through the center of the tip, the ferrule stiffness does not come into the equations.
Me, I just use a 19 oz cue from home (i.e. expendable) to keep the high forces from the joint and divots off the tip of my play cue. There is nothing special about this cue other than it came with my table, is straight, and has a hard tip on it which does and will hold chalk.
For those that like to drag the tip of the break cue along the clothe/felt:: A) please dont' B) round off the edge of the break tip to minimize the damage to the clothe/felt. So, unlike the play tip where you do want a nice crisp edge on the tip, on the break cue you want a (tight) rounded edge.