RCI tends to be more fluid in its trade power evaluations (and more restrictive in trading up) than II is. So, things that traded well last year in II probably will next year too. In RCI, it's less clear. But, if you spend a few months reading various timeshare boards, you will get a good sense for what things are more likely to have strong value year-in and year-out; and those coastal peak summer areas I mentioned are among them.
But, I have a slightly different take on it. I think it is a bad idea to buy a timeshare with trading as the primary goal, and it is downright foolish to buy a timeshare with trading into *a single resort or small system* (like DVC) as the primary goal. There are two reasons for this. One: it's really easy to buy a timeshare, but really hard to sell it. Two: the rules of exchange are changing all the time. In just the past 2 1/2 years, there have been three complete upheavals in how DVC exchanging has worked that have forced me to adjust my approach---switching from II to RCI, RCI's global re-valuation on 5/31/09, and the move to "points lite" on 11/15/10. I am about to see another in a week or two, when Wyndham owners get their own system-specific exchange portal.
With each of these, I've had to adjust my approach to exchanging into DVC. But, because I was buying intervals for their *general* value, rather than for a specific exchange purpose, I've been able to roll with the punches.
If you want more reasons not to buy specifically to exchange into DVC, talk to all the people who bought inexpensive II traders in 2008, or summer mountain weeks in RCI in early 2009. Those were both "sure things" at the time people were snapping them up, and useless for DVC within the next three to six months. With the 11/15 change, people are falling all over themselves to buy cheap lockout units in RCI. But, if DVC changes back to II, or RCI changes their exchange system again, those folks are back to square one, with an albatross that is hard to dispose of.
That's not to say that exchanging is a bad idea. I do it, and I enjoy it as a hobby. But, if I total up the amount of time I've spent keeping track of it all, I would probably have been better off using that time on billable work for a client, and just buying DVC points with the proceeds.