Man you are asking all the questions from 5 years ago.. Anyways, from what I hear aluminum is stiffer giving a potentially harsher ride. Aluminum is also newfangled and thus looked down upon by the old school types. Regarding track bikes, there have been a lot of nice steel ones around for a long time so I think people are just partial to it for sentimentalities sake or something along those lines. For fixies it was originally the thing to convert an old bike that wasn't going to do much else, and that bike is usually steel. Nowadays people convert newer bikes even though they probably still work..

If you actually have a decision to make I would say just ride whatever bikes you are considering and take the one you like the most. If you are worried about what others think just get steel because it is cooler. *shrug*