Under a cabinet, I don't think appearance is an issue. Plastic probably has an edge in longevity...as in foever, whereas a brass trap will corrode ...eventually.... A brass, or chrome plated, probably has a higher success rate of not leaking at initial install. Plastic can sometimes cause a problem, as you can only tighten the nuts so tight. I usually replace the nylon gaskets with rubber ones, coating them lightly with plumbers grease. These seem to seal up better.
If you are in MA, or some other states, you will be required to solder the trap arm to your waste stub. If your code allows it, I would cut that stub closer to the wall, solder on a slip joint trap adapter ( sometimes called a Desanko fitting).