If/Else is probably fine for this. You may not want to use it is you anticpate the number of individual project directories to grow very much though. This is because you want have a big long ugly list of if/elses. What I would do is to store the project dirs in a hash keyed by project title. Your form would submit this key and you could then set the directory by pulling the value from the hash. Your hash would look like this:

%projectdirs   = ( "hypnotron",    "/export0/hypnotron",
             "death_ray",    "/export0/death_ray" );
[download]

Retrieving from the hash is just this:
$directory=$projectdirs{$key}
Hope that helps. Good luck.