I own DVC, several Marriott weeks (now enrolled in points) and Bluegreen points. Each is great in their own way and each horrible at times as well. DVC ONLY makes sense for DVC resort stays. Marriott makes sense (in general) for Marriott stays, Marriott exchanges and easy general exchanges. Marriott trades only with II (with limited exceptions. BG and Wyndham are likely better values though not the high quality consistently of DVC or Marriott. Hilton is another great points option, Starwood has seen a lot of negative press and changes lately.
As for Marriott, there are 2 ways in. One is to buy fixed weeks and the other trust points. Points are really only available for retail direct from Marriott currently. I'd steer you away from points but still be very happy to see one who knows what they're getting, buy fixed weeks. One just needs to understand the limitations, namely, no points and little access to the newest resorts. Of course if one decides to buy a Marriott floating week, there are many variables to getting what works best for a given family. What resort, what season, unit size, price, dues, lockoff options, etc.
The bottom line is once you pull the trigger, things will change and almost always in a way negative to your situation. The trick is to account for as many variables as possible and to get in as cheaply as possible so that the ultimately risk is low. Also note that RCI points, Wyndham, BG, Worldmark, Club Intrawest and others that work with RCI will give you the potential of trading in to DVC but there are certainly no guarantees that one can do this consistently.