First, nunchaku training is much different than most people think. That is because most people are only taught part of the art of using them. There three parts of the arts are:
1) Spinning and striking …..the part most people learn
2) using the two sticks as if the were only one. Both sticks are held in one hand and used to block, strike, …etc.
3) The last part is the use of the nunchaku to trap wrist, joints, the neck, other weapons.
NOW to the type and size:
A good pair of nunchaku should have rope connecting the sticks. The gap between the sticks should be short. To measure the gap the nunchaku are hung over your wrist so that your palm is facing the floor. Ben your wrist until you can clamp the sticks between your thumb and little finger. If you can cross the sticks, the rope gap is to long. You should just barely be able to cross the sticks. Having the gap short allows you to properly do the wrist traps and other grappling techniques. It also makes the nunchaku easier to control.
The length of the sticks should be measured like this. With one arm bend at the elbow, place the nunchaku with the ends of the sticks in the crook of your elbow. The other end of the sticks should just reach the base of your fingers where they meet you hand.
The ideal type of sticks are hardwood, with eight flat sides and tapered.custom nunchucks
NOTE: the proper way to grip the nunchaku is like this. when gripping one of the sticks the little finger of the gripping hand should be two fingers width from the large end of the stick. Most people cheat by gripping in the middle of the stick or close to the rope.
Further Note: There are basically two different styles of using the nunchaku. There is the Okinawan method and the Chinese method. The Chinese method is the one most people think of when they think of using the weapon.
BTW- I learned my nunchaku from one of my instructors that had learned from Takio Yamura. Yamura lived in the U.S.A. for a short time. My instructor was the only one Yamura taught while he was in the U.S.A. He taught many traditional weapons to my Shihan. No kata was taught. Only weapon to weapon combat, done full power. I market a video on the basics of the nunchaku. In my video I teach the basic grips and measures I mentioned above. I then teach a traditional Okinawan Nunchaku kata. that is because it is just not possible to correct peoples form in combat over a video.
Hope this is helpful!
EDIT: you asked why they try to tell you chain is better. they are selling them. It is much easier and cheaper for them to sell the chain because it does not require as much woodwork or as good as wood. Metal and other materials are no better. Many martial arts sellers sell what ever they think that can make the most on. that is why they sell the crappy sai, graphite bo, chrome plates dull kama,….etc.