Do you already know asp and are considering php as well? It really boils down to personal preference, but since most scripts seem to be written in PHP, it would be the preferable language to learn. If you are doing all your own programming and not modifying other scripts, then the advantages of one language over the other are highly debatable. I've found the combination of object oriented PHP5 with the smarty templating system (smarty.php.net) superior to any asp templating solution I could find.
Hi Gilgalbiblewheel, I think the biggest difference you'll find is server support, php can run on on servers natively (ie apache,iis,lighthttpd etcetc) where as asp will only run natively on a windows machine. what gwkg said about advantages of the language, that's general personal choice and that question usally starts some "heated" debates about the subject