Marriott rules are more complicated than Starwood (which I know well) and they also seem more complicated than HGVC. HGVC is definitely the most resale friendly system of the 3. From what I've seen, IMHO, their quality is below Starwood (Westin Resorts) and HGVC. But, they have a lot more locations even though they have a large concentration in Hilton Head and Orlando.
I own a Marriott 2-BDRM Lock-off (L/O) week and I also own a HGVC worth 7K points.
1. The HGVC points system is nice and easy to use, although I don't like being limited to to a 9 month reservation window instead of 12/13 months (Marriott) or even 24 months (RCI).
2. I really like the Marriott preference, which always gives Marriott owners access to Marriott properties before others in II.
3. Each year, I L/O my Marriott week and deposit both weeks into II. This always gets me 2 weeks, which I use in Hawaii, for a 2-BDRM Marriott or Westin TS. Last year we even stayed in a 2-BDRM TS at the Marriott Aruba Surf Club.
4. From my experience, I'd say Marriott's are definitely of better quality than the HGVC's: granite counter tops, stainless steel appliances, seem to be maintained and upgraded more often, etc.
5. On Oahu, HGVC is on Waikiki Beach and Marriott is in Ko'Olina: both are nice and different. HGVC is on the Big Island. Marriott is on Maui and Kauai. I've stayed at all of the properties including the Westin on Maui which was super nice.
6. HGVC has properties in NYC, which we've stayed at. HGVC also has a TS in Miami Beach.
7. Marriott is in the Caribbean: St Thomas, St Kitts, & Aruba. HGVC is not.
8. Marriott also has TS's in Paris, Spain, and Thailand.
9. Marriott weeks uses II for all Marriott-to-Marriott exchanges, whereas all HGVC exchanges are internal.
10. You can use HGVC for Italy, Portugal, Scotland, & Mexico.
11. HGVC is good for short points stays, like Las Vegas New Years Eve, NYC, and the Lagoon Tower for a few days.
12. Marriott has a lot more locations along the East Coast.
Both systems are completely different, the resorts are different, and for the most part the locations are different.
I really like owning both systems and highly recommend it to others. I wouldn't say one is better than the other, I like them both, but would never buy into the Marriott points system.