If you plan to buy only one then you need to be very careful. I recommend to my students to invest in a good brand name. I prefer Saroyan brand because they carry a variety at reasonable prices. There are other quality brands but these are the ones I know best, so I will break them down for you. One of the things to consider when choosing a pair of zills is that they are are be played.
Most zills are constructed of brass, some are silver and others are a delicate mix of many metals.
You need to know that the higher the tone the longer the sound carries. When dancing consider the size of the area and how many people are in the space. If you dance to a crowded hall I recommend a high pitch pair, if you dance in a home then you will probably want a more mellow tone pair. Often how I feel also affects my selection of my zills. If I feel great and am doing a super fast piece then I want a medium to heavy weight zill.