[Info] Should I place landing page on unique or sub domains?

Hi,

I've always placed landing pages on its own unique domain...but I was wondering if conversion is still as good if I was to place my landing page on a sub-domain like trafficbooster.mywebsite.com OR mywebsite.com/trafficbooster.

The website is my own which is all about inteet marketing and I have a decent client base locally. Your thoughts?