I assume you are talking about the Marriott points program (you can also buy a Marriott week, but those don't come with points unless you pay big $$$ to enroll).
The difference between Marriott and Vistana is that with Vistana you are buying a deed to a specific property (similar to Hilton). That deed might come with points (StarOptions) at some resorts (called "mandatory resorts"). Vistana Villages and Westin Kierland are mandatory resorts. Click
here for a good overview of the Vistana system. You can get a good week with points for anywhere from $1000-$15,000. Maintenance fees on these would run around $1600-1800.
With Marriot points, you are just buying into a land trust. These points can be used to book at any of the Marriott properties. The buy-in can be pretty steep. To get a week on Maui in an Oceanfront unit, you would need around 6500-7800 points. Resale points can be picked up around $2 per point, but then Marriott adds a $3 fee (along with some others). So you would be looking at about a $32k buy-in minimum for 6500 points. Maintenace fees on 6500 points would be around $4k per year.