If it really is your first choice, you should definitely let them know. In my PS I did include a "why XXX" paragraph at the end. Each school to which I applied did have something specific that attracted me.
For my first choice (UNLV) - to which I was accepted and will be attending in < 30 days, I also called and asked for an earlier notification than they normally give. It may have been risky, but I am definitely a non-trad student and it worked for me. Not only was I accepted, but I also found out about a month earlier than I otherwise would have. I do think, however, that doing so limited my chances of receiving $$$. In fact, I received $0 scholarship and some with lower numbers received at least $omething. Nevertheless, $ was not really a concern for me. That school fit best with my life, and with my future goals. In asking for this earlier decision, I called the admissions office and was transferred to the director of admissions. I told him straight out that; I would most definitely attend if I were accepted. I told him of my personal circumstances (i.e. - an upcoming job transfer to the area, family migrating to the area, my affinity for the school, etc) In short; I guess you could say that I showed him that I would be committed to the school if they would give me a chance - without coming across as being desperate. He told me to fax him an additional letter to the effect of what I had told him.
At this school, a committee does make admission decisions, but I cannot help but think that this really did help me get in. If you are interested, I have posted my PS on a couple of blogs. (See http://mrsmittie.wordpress.com/
). You can also PM me if you want to talk.