If you buy DVC, it will cost a minimum of about $10,500 regardless if it's from Disney or from a resale. The cost has continued to increase since DVC began, and DVC memberships are pretty much protected from losing money--it's not guaranteed, but it hasn't happened so far. Once the price falls to a certain level (about $72 per point) Disney will buy it back, so it's a good investment. Once you are a member, your main yearly costs are dues. If you trade out, I believe there is a non refundable $75 fee, but that's it.
At Marriott, a buy-in costs about $18,000 for a week, but prices do vary. If you have to sell, watch out! You recoup only about 1/2 -2/3, again depending upon resort. Resales are CHEAP! If your friend wants to buy in Orlando, Marriott resales begin about $5500 and go up from there. Dues are $500-$700 or so per year. If you trade to ANY Marriott resort, there is a fee for that. $79 to Interval, and $121 if they get you a trade, and changing your ressie in any way means a fee, too ($50??) Marriott has lots of small fees for things that DVC members take for granted. Marriott also requires a week's stay, so if you want to visit Arizona, you will visit Arizona, you can't spend part of a week there, and then go to Colorado. Marriott is good if you want to STAY put for a week, but if you want to travel around, it's hard.
Your friends need to decide if staying on-property at Disney is important. If it is, by all means, buy Disney. If they don't care, maybe a Marriott resale is good for them. As you have heard before, buy at the place that you want to stay at. For Disney on-property, it's a must, and it's cheaper for the owner of a Marriott property, too.