Membership Site - Part of Main Site or Its Own Site?

I am thinking of starting a membership site/section as an addition to one of my sites. I was wondering if anyone had any input as to whether I should make it a part of my main site (it would fit in perfectly and not be out of place at all) or whether I should break it off completely and put it on its own domain (I have the perfect domain for it, by the way).

Of course I could always still advertise it on my main site even if I broke it off, but I was wondering if there were any serious pros and/or cons for either option that might not be so obvious?

The truth is the membership site might not ever get that big, but it seems to fill a little niche that hasn't really been filled. For this reason, I know that creating a new site will take more SEO effort if I want search engine traffic, but with this niche, there isn't likely to be much search engine traffic to go after. (It's something like some WSOs -- you would never search for what they're offering, but when you see what they're offering, you say, "Hey, yeah, good idea.")