There are many schools of thought on this subject. Here are some things to consider.
Aluminum usually weighs more than plastic.
Aluminum bends. Plastic breaks
Aluminum costs more than plastic (usually.)
There is a certain cool factor to anodized aluminum.
If you take the cool and cost factors out, it makes the decision a bit easier. I would prefer to have a plastic arm break so I know what to fix. If you bend an alum arm, you might not notice it. Then you end up fighting your setup trying to compensate for a bent part. I race in a very competitive club. In an environment like that, you need every advantage you can get. If you can save a few grams here and there, it makes your car more responsive and faster. This translates into a more competitive car.
It really depends on what you are doing with your car. If you are bashing and just want it to look cool and have everyone say "wow" when they see it, aluminum and titanium is the way to go. If you are racing, you probablt want to use aluminum more sparingly. Take a look at the top racers cars. I don't remember ever seeing aluminum arms on one. Sure those guys are better drivers than we are (well, me at least!) I hope that helps!
I plan to take my TA03F DJ edition out of mothball this weekend. My daughter wants to start racing and doesn't really like her TC3. I can't say that I blame her!
I picked up adjustable camber links for it so I can get it all set up for her. Should be fun!