We own at both. As menetioned, HGVC has a lot more flexibility for reservations. However, they have a lot fewer locations to choose from for internal exchanges. Even the locations they do have, many are affiliates which do not have 100% membership or availablity to HGVC members.
Marriott, on the other hand, has a huge selection of locations with top notch resorts. Those resorts generally enjoy prime locations.
As for not having to go through an exchange company with HGVC, that may be true but, you do not have total access to RCI without having to go through HGVC. I can do and see a lot online with my I.I. and Marriott. With HGVC, I really don't have online access. I like to do things myself and I don't care for having to call HGVC. The one time I wanted to exchange my Hilton week, I had to call them and then was only offered ONE choice in an overbuilt location (Branson). With my Marriott weeks I can see several choices for the same area. I've even snagged two bedroom units with my studio Marriott unit.
Hilton is a fine product but, for me, I prefer my Marriott weeks.