Personally, I prefer the subdomain method, and the reason is that on a subdomain I can pop that on a seperate server and just link the IP numbers rather than having to do rewrites and redirects on a folder method.
The advantage is that you can then spread your sites (or empire) over multiple machines which you can tailor to the needs of that task specifically. If your forum gets too popular, you just need to upgrade THAT server (or add more memory etc) rather than having to worry about moving ALL things.
The other item is that by spreading it over multiple machines, you're giving your users or clients multiple ways to get a hold of you should one of your servers go offline. This goes back to the old adage of "don't put all your eggs in one basket"
If you do decide to create a forum in a subdomain, make a 301 redirect on a folder for good measure