A Worldmark contract has historically held it's value quite well due to the (relatively) low maintenance fees, the flexibility of the system, etc. The challenge with Worldmark will be more the buy-in than the exit strategy, because Worldmark contracts are popular. It may cost about $3000 to buy a 10K credit contract, and that may or may not be enough credits to get a 2BR unit for a week. But no worry - once you're in the system, you have flexibility to buy additional credits without adding new contracts, so if you need a few more credits to make a reservation, you'll be able to do so.
Wyndham contract pricing is far more variable than Worldmark. Worldmark sits pretty consistently at .30/credit. A comparable Wyndham CWA contract - say about 231K points - may range from free to $1800 or so, including closing costs. The variability means if you buy low, you're likely okay - but if you buy high, you may have trouble getting out of it close to what you put in.
Worldmark has more presence on the west coast, Wyndham more on the east coast. They have a few areas in common. Moving forward - and this may not always be the case, but over the past 12-18 months seems to be the case - Wyndham as a corporation is putting the higher priced, higher demand areas in the Wyndham system.
You'll also want to figure the effective cost of your vacations within each system. With Worldmark, you can almost disregard the buy-in price, because you should be able to get at least 80-90% back out when you sell. With Wyndham, obviously it depends upon timing and price points, since it's so variable.
While you're vacationing, though, you can use a couple of benchmarks to help. There are some locations that are common between Wyndham and Worldmark and it is useful to see how much in maintenance fees each unit "cost" based on the points required to stay there. Let's check a 2BR at Steamboat Springs:
Worldmark - Red - 10,000 credits = $685/annual dues (if you have a single 10K contract)
Wyndham - Prime - 231K points = $1259/annual dues (if you are using a CWA contract at $5.45/K).
Obviously, not a favorable comparison for Wyndham. However, let's try a Ron Parise special at Avenue Plaza:
Worldmark studio 9000 credits = $616
Wyndham studio 126K credits = $687
Much, much closer. (There are some Worldmark studios that are "hotel" studios that are 7000 credits, however.)
One final comparison: Indio
Worldmark 2br Red = 11000 credits = $753
Wyndham 2BR Deluxe Prime = 180.4K points = $983
So in determining which system, you need to figure which has the resorts you want to go to first... the financial considerations can follow.
Good luck.