If you want to find a topic about which most violinists will disagree, the subject of shoulder rests should be high on your list! Every violinist has their own preference about which shoulder rest to use, if at all. Shoulder rests have only been in mainstream use since the 1950s, and some players still view them with suspicion!
Some people argue that a shoulder rest can reduce your agility, and that the violin becomes stable in relation to your body position - it cannot be moved around as easily. Some types of shoulder rest can also cause a slight muffling of the sound. But generally these are not significant enough issues, and nowadays many people prefer to use a shoulder rest (although it is still common for players of baroque violin not to use one).
The purpose of a shoulder rest is to give you more comfort and flexibility when you are playing. This is particularly the case if you are practicing, rehearsing or performing for a long period of time. If a shoulder rest is correctly adjusted, this should also improve your technique and prevent any long term neck problems.
Choosing a shoulder rest is more about finding one that is appopriate to the shape of your own body than that of your violin. The lenght of your neck and the breadth of your shoulders are the most important factors to consider.
You will also need to work out exactly where the right playing position for your violin is in relation to your body. To do this, hold the instrument as if are about to play. Ensure that your shoulders are relaxed, and keep your neck straight, without projecting your head forwards. Once you're in this stable position, you simply need to identify which shoulder rest fits best between the violin and your collar bone.
It is very important that you try out different types of shoulder rest to find out what works most effectively for you. If a music shop gives you a shoulder rest and expects you to buy it without trying it out, refuse! Make sure that you allow plenty of time and insist that a shop gives you the opportunity to try out different types of rest before making your purchase.
Finding the most comfortable shoulder rest is a bit like looking for a new technique in your playing: you need to work out what is most comfortable for you, that also gives you the most flexibility of movement. Experiment with different shapes, and if necessary consider bending the rest into a shape that feels comfortable, or try using a cloth, sponge, or other adaptation to see if you get a better result.
Above all, remember that a shoulder rest is a piece of equipment that is extremely personal to you. By trial and error, and with plenty of persistence, you will find the setup that is right for you.