I've got a fairly high page ranked site: www.mainsite.com and am
planning to have another site or folder for campaign landing pages.
Either I will do www.mainsite.com/specificlandingpage or
genericname.mainsite.com/specificlandingpage

Each site will have it's own site map, the campaign landing pages will
have unique content and send outbound links to the mainsite, and
www.mainsite.com will be the preferred domain (to avoid duplicate
listings).

The question I have is there an advantage to doing things one way vs.
another way. From what I have read, it seems like there isn't. However
you have to be more careful subdomains to avoid looking like spam
sites.

If there is no advantage to either, and we wanted to use subdomains
just for a maintenance and reporting advantage, what are the best
practices (unique content, link sharing, etc.)?

Any guidance would be appreciated.

Thanks,