Keep in mind that the travel demand index indicates relative demand for a given region, not for a specific resort and is not applicable for comparisons between resorts/regions. That is, for example, a 150 Orlando week means that it is a high demand Orlando week and, while likely a Platinum week, does not mean it has the same trade power as a winter Caribbean week that is valued at 110 or 120. So the demand index is useful when deciding what weeks to book for trading, but doesn't indicate relative trade value between resorts.
If you buy a floating Platinum week you are guaranteed a reservation within the Platinum period at your home resort (as long as making the request before the season has begun for the year in question). Due to relative trade value between certain resorts, a Platinum 2BR in one resort may not be strong enough to trade for a Plat. 2BR in another locale, esp. during peak time, of course depending upon which resorts are in questions. Winter Caribbean and ski weeks and Hawaii weeks, esp. over the summers/school vacations are your hardest trades and, similarly, will be your best traders. Summer HH weeks, summer NCV and a few others will also be great traders.
Besides buying where you'd like to travel, consider what locations and during what periods you want to exchange into. If you like to travel a bit off season your needs will be different than someone locked into prime weeks (school vacations, etc.). If you always want a 2BR you will need a stronger trader than if 1BR or studio units are fine with you. If you are looking to regularly trade into peak resorts during peak periods, make sure you buy a comparable trader; buying with the expectation of always trading up is likely to make you an unhappy timeshare owner. Of course, there will be times when you "luck out" and can trade a less desirable week for a top week, but I wouldn't buy expecting those trades regularly unless you plan on trading less than 59 days before travel (during Flexchange); in that case, you don't need a Plat. week to get top weeks elsewhere.
So, as you decide where to buy, besides thinking about where/when you like to go, think about unit size needed/wanted and how far in advance you like to make plans.