No personal experience with timesharing in either Newport or Cape Cod. But from what I've read over the years (and from what I remember of New Englanders' vacation habits when we lived in CT), both are very tough trades in prime summer (last weekend in June through first weekend in August). However, I would think a prime beach week would surpass a prime sightseeing week. Also, if you're talking about the rest of June or August, the beach week is hands-down in greater demand. The month of May could even be slightly better in Newport than in Cape Cod, not sure. Offseason in both places are easy trades. These are just my general impressions.
As for which resort and why, brand name or not, I think it really depends on the reason for getting the week in the first place. If your friend wants to use the week, it's really worth traveling to the area for fact-finding, visiting multiple resorts and seeing firsthand what the units look like, location and amenities. Even selecting the specific units with their views can matter to a home resort user. If your friend is mainly interested in exchanging and/or renting it out, with possible occasional usage, then a brand name can help in making it easier to trade into sister resorts or get attention for the rental. Make sure the resort trades through the preferred exchange company.
When buying in Newport, the vast majority of units were originally sold as fixed or floating weeks, not as points. So it's possible to find weeks at all of them, for sale by owners. Ask at the resorts themselves if they have resale listings or a bulletin. Newport's is a great place to vacation - hope your friend enjoys their hunt.