Differences of strings and machines
Stringing machines features
A stringing machine need about 25 kg for tennis rackets and 11 kg for badminton rackets, maybe some more or less what you prefer. The power could be made by a weight, a manual handle or electric motor. The leveler models have a weight on a bar which could be set higher or lower at the bar for the right kg's. The machines with electric motor could be linear or turning. The linear motor don't bend the string so could be used for every string type. Older style stringing machines need a calibrator for "tuning" the right kg's. The best machines have measurement for the amount of kg's and continuous update the right kg's (continuous pull). Even though some machines could set the kg's to an accuracy of 0.1kg, the real kg's is more dependent to the way of stringing.
For pulling the string, the string should be locked easily before the machine pulls the string at the right kg's. In the past, strings should be turned around some holder with rubber or turned around 2 half circle parts where the string at the end should put in between the 2 parts that come closer to lock the string. Now almost all machines use 2 metal bars that glide left/right and come more close to each other when fastening so it then holds the string. This don't hurt the string and there is also no risk to cut the string (when putting wrong in-between) like with the 2 separate round circle parts.
The racket holder often uses 2 inside bars at the begin and top of the racket-head. Then, also at the outside of the racket head, bars clamp the racket head from the outside. The outside bars, often are "V" formed with a smaller "v" part for badminton rackets. Because some tennis heads could be very big, often the top and bottom part of the racket holder could be set at a bigger distance from each other. The 6 metal bars, for clamping the racket, need some (plastic) protection for not hurt the racket.
String clamps are sometimes fixed to the racket holder via a bar where these glide up and down, then you only need to clamp the string in one action, these are often expensive machines. Most machines have a holder for the clamps which should be fixed after moving at the right position and then the clamp can lock the string, so 2 actions. The holder and clamp should fit perfectly, otherwise there will be some space and the clamp then moves too much when locking the string. The string clamps should not hurt the string and also not loose the string. Always the clamps could be set more/less tight because all strings are different. To not hurt the strings, the clamp could have many fingers (5 preferable) to divide the power on the string. Also the clamp could have bigger fingers so it don't show a print of the finger in the string.
Because you always loose power when making the knot, it is nice when there is a button to put some extra kg's before knotting. Some machines have a fixed extra kg's for making the knot, some you can decide yourself how much extra you want. More recent machines have a "pre-stretch" function. This adds always some extra kg's when stringing, before it goes back to the set kg's. This is very useful to overcome the friction of the string when going outside the racket. In the past, you had to do this with your hand power to help the string a little with the friction of the holes. "Constant pull" is a feature that the machine continuous keeps the right kg's. Also some machines need to be calibrated with a tool so that the machine pulls exactly the right kg's. If you only string for yourself, this isn't very important because you know which kg's you prefer, even though this might not be exactly the real kg's. The S213 don't need to be calibrated because it shows always the right kg's at the display. Some machines you can save your preferred kg's and some more but this is not so important. For linear machines it is very important that the machine warn you when the pulling motor is at the end (while not reaching the desired kg's), this can happen when the strings is very flexible. You then need to string another time.
Important is the right height of the machine. When you have a "table model", you can put it somewhere on a table for the right length of the "stringing person". Also a foot could be used for the right height but when the height of the foot is fixed, it should match for the person who strings. A variable foot is preferable when several sized persons string with the machine. A table model is great for taking in your car when moving to tournaments. Some machines can't be used both as table model and foot-model, you should realize before buying.
The clamps are often in a holder. Important parts for this are: these fit exactly in the holder so they don't move a bit, the "fingers" for grab the string shouldn't hurt the string and also hold the string well and so should be flexible distance for smaller/bigger sized strings.