K2Quick
TUG Member
WM will likely not be the absolute cheapest trader in II. Looking for the cheapest can be a really bad idea, though. Generally speaking the resorts with the absolute lowest maintenance fees are lousy and have lousy trading power. Also, don't forget that you might well figure out that the timeshare thing isn't for you a few years in. I wouldn't personally buy anything that people have have a hard time giving away or have to pay people to take off their hands because that will likely be you somewhere down the road. Buy something that will retain at least some value so you can exit easily if you want to.
If you're looking strictly to exchange, find out what you most want to exchange into and then figure out what the best trading option is. There are a couple of systems (Vistana and Marriott) that have internal preference where their owners see II inventory several weeks before general II members. If you want to get the harder-to-get units in those systems, it can be better to own within those systems (but usually costlier).
When I bought WM, I thought I'd be exchanging more than using the WM system internally. But as the kids got a bit older and more tied to the school calendar, we gravitated towards using the WM system almost exclusively.