this is a common question, and many have their own opinions that may be different than mine. If you do a search through the various forums (in the search screen there's an option for "all forums") you'll find a lot of discussion on this.
The perl version running under mod_perl is roughly equivalent to speed as the regular php version. The perl version does run a bit slower, but it's hardly noticeable on most sites. If you expect 25,000 users a week to your site, then it might matter to you to go with the php version for speed.
As far as setup, php would probably be easier just for the sheer fact that there's less to install. A very important exception is the cron tasks. By cron tasks I mean anything that you set up to be automatically sent at times you specify. Examples of this would be subscriptions to your forums that your users sign up to get (provided you allow this). Erm, Poil talked about php allowing cron tasks via lynx, I believe... you'd want to get with him or someone in the know to get more info on that though, I'm not as knowledgable yet on php as I'd like to be.
I like Perl because I'm getting to know the language and can manipulate it better. There are more hacks and plug-ins provided with the perl version, mostly because it's been around a lot longer than the php version has.
If you're extra shaky on setting stuff up (cgi scripts in particular), perhaps the php version is for you. As far as W3T's site goes, there's better documentation (at least in the support library, what's included in the distros is different) for the Perl version simply because I haven't gotten around to doing the php version. I really want to get on that and hopefully will have the windows versions of php done by this weekend.
Sorry for the long post. I'd go with Perl, personally. There's a few more steps involved in set up, but they're not difficult steps and you may find you have more options available to you with the Perl version. That is quickly changing though so it's up to you.
Not sure if that helped any. []/testimages/icons/crazy.gif[/]
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