At one time I played with a 21 oz cue. This was when I was hitting each shot hard. (Poke and hope!)
Then I learned that the idea was to leave the cue ball in a spot after the hit so you can shoot in the next ball, and sometimes a very soft hit was the best thing. Also some safety shots require an extremely soft hit. It was easier for me to make these slow soft hits with a lighter cue.
But what weight cue should I get?
I searched the internet and found some lists of pro players cues and their weights. 19.5 oz seemed to be a common weight for the pros. So I got a 19.5 oz cue. And this weight seems pretty good to me. Kind of in the middle. I can shoot very slow shots and very fast shots.
The next thing I learned was speed control. Being able to shoot a shot and get the cue ball to roll a specific distance after the shot (to leave the cue ball in a good spot for my next shot).
It seems to me that for *accurate* speed control - being able to judge how far the cue ball will roll after the hit, if I *always* play with the same weight cue, then it will be easier to learn this - be able to predict how far the cue ball will travel after the hit.
So I *always* play with a 19.5 oz cue. And I'm getting to be pretty good at shooting a shot and getting the cue ball to roll exactly where I want it to stop.
Next I learned that some cues are not marked correctly as to their weight. I learned this when I purchased an accurate scale and weighed various cues I have for my pool table at home and weighed friends cues. So a cue may say 19 oz and actually be 18 or 20 oz! Then you go and buy a new cue which really is 19 oz and suddenly your speed control is off.
Also note that better quality cues have weight bolts in the butt. These can be changed to adjust the weight of the cue.
So bottom line, I would suggest finding the right weight cue for yourself (people are different, what works best for me may not work best for you), then stick with that weight of cue. Also verify the true weight of the cue with an accurate scale. Then if in the future you buy a new cue, you can get another the same exact weight.
Here is an example of an accurate scale...
(KD-200-110)
http://www.scalesgalore.com/tanitakd200.htm