Most of us have heard of some timeshare horror stories. That's one reason many of us waited so long before jumping into DVC. What's the best way to do research for those who are thinking of branching out beyond DVC (e.g. Marriott timeshares)? Redweek? Tugbbs? Ask questions here on the dis?
First I'd do some soul searching. Buying DVC AND another timeshare can be a great choice for many but it requires additional volume. In addition one needs to be willing to get an education up front (it seems you are), plan ahead even more than DVC, pay cash, be OK with the compromises and risks of timeshares and that there is a timeshare or timeshare system that will get you where you want to go. Getting you there and having a timeshare there are not the same thing.
I'd say if all of those fit, one needs at least the equivalent of a 2 BR yearly or similar volume, more is better. As for where to get info, the things you mentioned are ALL a good choice. Redweeks, the exchange company directories (accessible to non members), timeshare company websites and the like are great places to get a feel for what/where on the timeshares. TUG is likely the best single place, esp if one has a paid membership to get access to reviews/ratings. I'd also add timesharing today and there are a couple of other sites similar to TUG. Email lists, Facebook and Yahoo groups are also great once you start narrowing down. DIS and similar are great places to start not because you'll get a ton of info but because it'll be people with perspective on DVC and other timeshares and it'll help you focus in on the best choices.
So assuming it seems to be a good choice to you, decide where you want to go and how you'll use it. See who has options that work for where and how you need. In general I think most people are best off with a mini system or similar like Bluegreen, Wyndham, Worldmark, Marriott, Hilton, Hyatt or similar; esp for DVC members due to the similarities. Once you have sufficient knowledge and know what fits for your situation, the best choices will be fairly obvious.
Make sure you learn about club vs associate or affiliate resorts which are linked but not legally a part of the group so they could go away and often have limited availability, esp for the points systems. Investigate the likelihood of getting what you want even if you plan ahead and if there is a VIP system or other reservation preference that will affect the change of you getting what you want. VIP systems are almost never worth paying for. Here are a few thoughts on some of the systems.
Wyndham: likely the best mix of resorts, quality, price, volume and flexibility. Not all resorts are high quality but most are though not necessarily consistently up to Marriott/DVC standards.
Bluegreen: likely the best value if it works esp for exchanging and esp where considerable volume is involved. A lot of resorts but not as many as Wyndham and some are not great and most not to the level of DVC.
Marriott Trust points: Flexible, fairly expensive like DVC. Marriott has a bunch of top resorts in great places.
Marriott Weeks: Even though this feels less flexible, it can actually be the best choice for many. Buying 2 weeks that can be reserved together can be the best option and they don't have to be the same resort or same location. For example, I own 12 Marriott weeks and 2 are lockoff's. So a month or so ago I reserved them all. I separated the lockoff's into 4 weeks and reserved them consecutively then the next 2 weeks in HH (multiple villas) then HI (Oahu). So using the 13 month reservation option and the ability to reserve consecutively (& concurrently), I was able to reserve up to 15 months out and guarantee my HHI weeks which was my main goal.
Worldmark, Hyatt, Hilton: I don't know as well. Worldmark is more out west and is tied to Wyndham currently so there are crossover options.
Lastly, make sure you know things can and WILL change over time so have a healthy dose of cynicism and be able to separate the sales process and the system itself, esp for some. Just spend some time, it'll feel overwhelming at first but things will become more clear as you learn.