Have been struggling with this at my latest gig, normally we do Hybrid but this was a spin off. 6000 Seats without Hybrid is turning out to be an absolute administrative pain for groups, addresses, aliases, dynamic groups, etc. I don't believe there is a document anywhere that will give you absolute pros and cons though. Be aware that a lot of the benefits with Hybrid are that objects are 'mastered' on premises which stamps them with an identifier in 365 so they can be administered from the on premises exchange. Introducing Hybrid later on/now will not necessarily allow you to administer any existing 365 based objects, only new ones. If i find a good grief counsellor i'll let you know.