Multisite or Multiple Installation?

I’ve been struggling with this for a while. Here is what I am trying to do:

My site is a listing directory(theme1) an online magazine (theme2) and further down the line a community (BBpress theme3) I’m not enough of a wiz to fuse all this into one theme as each have multiple custom functions.

The biggest problem I have with this is that it needs to be a membership site and ideally when logged into one site, the user is also logged into the other. A good example of this is Themeforest.net once logged in the user remains logged in all their other sites.

Any feedback/views/opinions would be highly appreciated.