Key Factors To Consider
I own within the HGVC and Starwood/Westin systems. In both cases, my home resorts are in Hawaii (one on the Big Island and the other on Kauai). I too share Ron's most important piece of advice: focus on the location of your home resort first. In fact, my key considerations in order of importance would look something like this:
1) Location. Make sure you're buying in a place where you truly love to vacation, want to vacation in the future, and don't see yourself growing tired of visiting for many years to come. In this regard, think of your stage of life as well. For example, buying a timeshare unit in Las Vegas probably isn't the best choice if you have young children, even if you and your significant other love it (unless you determine you don't want your kids to vacation with you). On the other hand, buying in Orlando is great if you have young kids, but if you're soon going to become empty-nesters, do you really envision yourself going to DisneyWorld every year? One more thought on location: Do NOT get caught up in "trade value" in order to buy a location you really don't care for but which has good trade value. I'm not saying it's not a consideration, but IMO it should be far down the priority list.
2) Cost of ownership. Too often, buyers focus on the upfront cost of the transaction as the major consideration. It's not. Ongoing maintenance fees are a major consideration, but there are others. For example, what is the annual cost to travel to your desired location? In the case of Hawaii, airfares have risen steeply in recent years as demand for Hawaiian vacations has grown tremendously--driven primarily by Asian visitors from Japan, Korea, and China. Occupancy rates in Hawaii are at near-record levels, and flights to and from Hawaii are near-capacity. This cost--the cost of actual travel to, from, and while visiting your timeshare location--is too often overlooked but should be a critical part of your decision process.
3) Ease of use and access to the resort system. In this respect, I give HGVC very high marks. Their online reservation system is outstanding, easy to use, and generally very accessible with enough advance planning. Reservation fees, cancellation fees, points rescue, points borrowing, and other features provide tremendous flexibility at a good value to the owner. I'm not familiar with the reservation systems for Marriott or Hyatt, but HGVC is superior to Westin/Starwood, even after Starwood finally joined the 21st century earlier this year by providing an online reservation process.
4) Potential resale value. Other TUGgers might put this factor as a higher priority, but in my mind--if you've done your research and have determined the frequency, location, and system you're interested in buying into to best fit your budget, family needs, and long-term goals, then buying something with an eye toward its value when you decide to get rid of it should be less important. That said, it is very true that specific resorts and systems can and do change over time. That's one reason I have stuck with two of the most reputable systems. My theory is that the major hospitality companies do not want their reputations or image sullied by having a timeshare division that degrades its brand, and so Hilton, Marriott, Westin/Sheraton, Hyatt, and Disney will fight like tooth and nail to uphold the quality of their timeshare resort offerings with tight quality standards. With regard to resale, remember that if you buy in an overdeveloped location (Vegas and Orlando come to mind), resale value will be very negatively impacted.
5) Desired level of "luxury". Like everything in life, the more "goodies" you want, the more you're going to be paying for it. For example, maintenance fees at the Westin resorts are among the highest in the industry, but the resorts are first-class, well-maintained, and greatly enhance the overall vacation experience. Lizap ranked Westin number 1 overall, and I would agree with that. I don't own at Marriott, Hyatt, Disney, or WorldMark, but I have stayed at these concepts through exchanges in the past and find Westin to be number one. But if you're willing to give up a little in terms of luxury and pampering, I think you can do very well at Marriott and HGVC with generally cheaper MF's (by several hundred dollars per year).
I know these factors don't answer your specific questions, but I thought I'd pass along some hard-earned knowledge that should help you in your decision-making.
. My Wyndham I basically gave away (because I basically got it for free) and it was painless. However, a sale of half of my Worldmark points took months to find a legitimate buyer and I ended up getting much less than I expected. But I sold it so I could buy my first HGVC. 
