Brass vs Steel

I have a steel roller trigger in one of mine. It does make for a far better trigger than the stock trigger. Whatever material you choose, make sure that the roller is lubed as it is a rotating bearing. If the hole in the roller is not perfectly centered it will tend to wear a flat spot and brass will be far more prone to do so than steel. Personally I will always choose steel over brass or aluminum for material on another steel moving part.
 
One advantage for adding a brass part somewhere in the mix is it is a good compromise between hard enough to do the job well, yet soft enough to ensure that the other mating part/s will NEVER wear. ie: the brass part can be "sacrificial" If it is also significantly less pricey than the mating surfaces, this is a good choice.

I have no first hand experience with this specifically with triggers, but in the machinery I work with it is a constant. Also for 1911 pistols, i prefer Brass grip bushings for this reason.