HGV is far simpler than Marriott. Marriott is a far larger organization with more members and resorts. They trade in Interval International (II), where HGV trades in RCI. Marriott bought Vistana and Starwood, and many people are unhappy with how the integration, or more accurately, the lack of integration has gone. Google "Bonvoyed" which is the term Marriott people use to describe being screwed over by Marriott. On the upside, if you know the ins and outs and play the game well, it provides opportunities that could work well for savvy owners. I wouldn't touch them, but that's me. You may want to work through on paper how a purchase works, the resorts you want, the time period allowed to make reservations, the MF's, the points and membership types needed, etc. Its complicated so taking the time to figure it out ahead of a purchase is critical.
https://tug2.net/marriott-vacation-club/marriott-vacation-club-timeshare-points-information.html
I own both HGVC and Marriott. Both Hawaii. With Marriott I do not have DC points, just an EOY 2-BR OF at Maui Napili. On secondary market, acquisition prices for me were about the same: 2-BR OF at Lagoon (9600 pts) was about the same price as a 2-BR OF at Maui Napili (Marriott is actually about $2k more). But, no doubt, the OBVIOUS difference is that Marriott MF fees approach double those of Hilton.
However, as an offset, the Marriott 2-BR OF in Maui is a lock-off and can be used as a 1-BR OF (includes full kitchen and washer/dryer) for a week and an OF studio for a week (kitchenette and no washer/dryer), so I get two weeks in Maui for the MF of one week, if I want. Or I can rent the studio out and pretty much pay my MF, making my week in a 1-BR OF essentially free. That is a notable advantage for Marriott.
In Hilton's favor is that, because my Marriott week is not "enrolled (i.e., can't be used as points)," I only have two options -- use the week where I own it, or trade. Hilton, on the other hand, has the points that can be used quite easily and pretty much at any HCV location, reserved 9 months ahead. So, with the HGV points, you don't have to buy expensive Hawaii, you can be inexpensive and use the points anywhere. You just give up a couple of prime weeks that may not be available via points like X-mas and spring break. But otherwise, the HGV points are infinitely more flexible.
Each also has a "cash" option. Marriott gives 25% off the "rack" price to all owners that rent at a Marriott Vacation Club location through Marriott.com Hilton has a much bigger discount, but only 30 days before check-out. So, it's kind of a last-minute what's available kind of thing.
I cannot imagine a compelling reason to buy direct over secondary market. Marriott gave me the "full monty" sales pitch to turn my EOY week into an annual enrolled week that can be used as points. Cost would be about $40,000. Since I can buy an EOY for about $20k or less, I saw that as paying an additional $20k. What would I get for that? Well, I could use it as points and go elsewhere. And I could use it as points and not be bound to a Friday or Saturday check-in, allowing travel at mid-week rates. Since Hawaii will almost always be where we go, it wasn't worth $20k for the option to check in on a different day. I just go two days earlier and rent for two days!