For a quick overview of the anatomy of a brush, different brush materials, and different brush properties,
this article should hopefully be helpful.
As I said earlier, some folks swear by syntehics for metals (their bristles break less and clean better), but I find that the brushes lose their points quite quickly (the individual hairs split or go 'fuzzy' at the tips usually). The tradeoff for me between a cheaper natural brush and a synthetic one is that I get a better result but wear the brushes out faster; since the whole point of any miniature painting brush is to get the model painted, I choose the better model result over the better brush wear. Like most plastic techologies though, synthetics are much better now than they used to be.
Malakai does make a good point though about different brushes for different jobs (aside from the more obvious 'bigger brushes for bigger areas', etc). I prefer a longer brush for detail work because the paint flows better to the small areas (hence my choice of W&N7s), but a lot of people prefer shorter brushes because the tip is stiffer and easier to control.
For fine edging, lining or flat panel work, you might find a flat brush more helpful than a round, as the shape of the brush can be guided by the edge you are looking to paint.
Ultimately, once you get past cost and wear, brushes of any type will suit people differently for a whole slew of reasons; painting styles, environments, and people's personal ergonomics are very varied afterall. The best way to find what works for you is to take the plunge and buy a few brushes of different types and qualities and work out what you like most.
