I use whichever I happen to type while I'm scripting. I never really think about which I'm using, looking through my scripts I've got a fairly even amount of each.
As far as parsing speed goes, # was about 0.0000025 seconds faster to evaluate in a quick benchmark I did. That means the time it'd take to just go through a single script file and change all uses of $chan to # would probably be about 1000 times more than all the time saved from evaluation speed as a result of the change in the entire lifetime of the script.
