A good break is a huge "advantage" over the other players that don't have what it takes as far as consistency, control, and power/speed go's. It's not only the opening break shot, on the break is playing to pocket either the head ball or a corner ball plus a few extras as far as 9-ball go's. But in the same token playing for shape off of the break by controlling the cue ball leaving it towards the center of the table. This gives the player better odds on position of a ball for the next shot.  Strickly keeping it as simple as a player can. Power or speed is very important but not as important as controlling the cue ball and better yet consistency is the key factor here overall. To develope a power break shot you must first learn to master tip position, this is based on muscle memeroy that the mind accumealates over a period of time and practice, or some kind of instinct. Over the many years of playing the game i've wittness numerous attempts of a player trying to perfect the power break without having the proper basic funtamentals that this requires leading to a miscue or jumping the cue off the table. Whats so good about the break if a player lacks  these basic fundamental (stance, hand bridge, tip position, stroke, speed/power, control, and consistency. without this your dead in the water, and the break becomes the players disadvantage, thus giving the  aposing player the opperternity of advantage. If a player has good funtamentals and masters tip position into their stroke ( meaning pin point accrussey on the cue ball at high speed ) then the power/speed will follow. Then it's a matter of controlling the cue ball and accrusey, and then consistency.

The player on the break should alter (shift) their body weight slightly towards thier front upper body. Also the players cue tip should be roughly about an 1/8 of a inch from the cue ball and at the same time their arm should be parralle to the ground and a 90 degrees off the elbow at the bottom of the pendelum arch of their stroke. The stance should form a 45 degrees between the two feet and the hand bridge placement. also note: if your right handed then your left foot should be pointing towards the "line of aim"  the direction that the cue ball path will follow. 

Using a break cue is key since every time the cue tip makes contact with the cue ball it changes the shape of your cue tip., with every hit. the more times you shape your tip the sooner you;ll be replaceing it. the tip will shape it self over game play according how the player plays and how much english they apply to the cue ball will determine the outcome of their tip shape. Other wards try not to use your playing cue for a the break cue.

What weight size is best for breaking? most players use a lighter cue other than the one they shoot with. Since the speed inwhich the cue stick comes through on the ball logically has a higher speed at point of contact rather than stroking a cue that weighs 2000 lbs and only traverling at maybe 1 mph.  Thats breaking in a nutshell.