I would start by making decisions to include:
- Where you want to go repeatedly.
- How flexible you can be.
- How far in advance can/will you plan.
- Budget.
- Usual length of trips.
- Unit size normally needed.
- How particular you are. Some people are more resort quality oriented and some more location oriented.
- Budget.
- Financial stability.
The answers to these and similar questions will give you an idea of what is best for you, if anything. The preferred locations will go a long way toward helping you decide which is the best system for you, it may or may not be Marriott. There are cheaper and more flexible systems (Worldmark, Diamond, Bluegreen, Wyndham, etc) and all can be great but they have their own compromises and limitations. Each one can be the best choice for a subset of people and bad for others. Specific to Marriott the main choices are whether to buy weeks for direct usage, weeks for exchanging, Trust points or weeks and get them enrolled (several ways to do so). If enrolled you might have the best of both weeks and points but at a significant cost. Points and enrolled weeks will come with a multiple 5 figure price tag. And since you own with Sheraton you could wait and see what enrollment options you might have later.
If your choices lead to to where exchanging is feasible and you make good choices on the acquisition, the price for 3-4 weeks could be as little a $1-2K up front and yearly $3K (2 lockoff units at a reasonable priced resort) plus an II membership if you don't have one already you can use, exchange fees, unit upgrade fees and lockoff fees would add another $1K or so yearly. Points for 3-4 weeks resale is likely somewhere in the range of $50-60K (15K points) right now but historically would have been higher. Weeks for specific destinations to use without routine exchanging will vary dramatically depending on the resort and destinations and could be as low as a few thousand $$$ for Orlando/Branson/Williamsburg/LV or as much as six figures for fixed ski weeks/Maui and the like. Weeks that are enrolled if you plan it on the front end could be as low as $60-80K (very roughly) but early 6 figures for the volume in question. Of course there are lots of in variations.