Topic: Subfolder or subdomain ?

Hi everyone.
I'm building a forum as an internship project here at the web agency dating sider. The webite is in php and html. We want a forum for people to disuss different options etc. We cant decide on putting the forum on a sub domain or in a sub folder as forum.datingpilot.dk or datingpilot.dk/forum - what would be best praktice?

I hope someone can help me make the right decision smile