Well, from what I've heard over the years - weight plays no part in how a cue effects the cueball. Assuming we're not talking about weight in the end of a shaft. Predator has claimed that a dime-radius tip deflects less than a nickel-radius tip shape. I've found, and many agree, that a soft tip makes it "easier" to impart more spin on the cue ball. I'm not saying a soft tip makes MORE spin, but it seems it makes putting a given amount of spin on a ball easier. This doesn't matter much, as your stroke varies more than the difference between the tips. Predator, and some other makers, claim that their designs impart more spin on a ball when all other factors are equal - such as tip placement, speed of stroke, etc...This might be true. Until there is some independent verification (maybe there has been and I dunno about it)..it isn't so easy to swallow what every cuemaker out there has to pitch.
As for weight, 19oz-19.5 seems to be the weight most pros use, and there's 2 theories as for break-cue weight.
There's the ligher is better. Which it techinically is, since a lighter cue can be swung faster, thus generating more cue-ball speed. More speed = more power. However, not all top players opt for a ligher break cue. Instead, they get a break cue with the same taper, the same balance, and the same weight as their playing cue - except with a really, really hard tip. It is more important to some players to maintain the same feel/balance/weight with their playing cue for accuracy than it is to generate a slight bit more speed by going to a lighter break cue.
To each his own. Note that the differences here are so minimal, they should be of little interest in seriously improving one's game. It's interesting for pool junkies, but if you're searching for a magic wand, or the excalibur of cues...forget it.