Hilton has a lot more flexibility in terms of the length of stay...and it is easier to book at the various resorts because you can do it in-house through Hilton rather than having to go through an external exchange company. This is Hilton's one major advantage.
In contrast, all Marriott exchanges go through Interval International...and they are all week-for-week exchanges. However, Marriott has lots more resorts and the resorts are nicer. Marriott has the edge in terms of architecture, grounds, decor, locations, etc. It's a better system.
Steve