The metal winding may be of aluminum, chrome, steel or silver. The synthetic core is often of perlon. These are louder, more responsive, and less stretchy than all of the above, but lacking in richness of sound. Used often for small student instruments, unusually dark sounding instruments, or for the distinctive sound of fiddle music. Violin E strings are usually of plain steel, steel wound with aluminum or gold plated.
Thick strings are generally louder.
Thin strings are more responsive, but softer. The answer to this question is very subjective. It depends on the personal taste of the musician, as well as on the innate qualities of the instrument. Experimentation will determine which is best for you. Some instruments may require a mixture of string types or gauges.