Whatever you decide, don't put all of your future vacation dollars into timesharing. This will give you the flexibility in the future to choose the best way to visit your desired destinations. There might not be a timeshare at the location that you want to visit or if there is a timeshare it may be very hard to get an exchange. You may also want to visit a destination in an entirely different way via a cruise, hotel/resort stay, special discounted travel package, tour company, etc....
As DeniseM stated above
(1) exchanging is kind of a crap shoot (available is based on the weeks that have been deposited into the exchange companies; availability varies; there are no guarantees that the time you want to visit will be available; high demand locations during high peak periods will have the most competition for timeshare stays)
(2) Owner's priority at their home resort (exchangers may not get the best rooms available)
(3) The most successful exchangers are very flexible and can travel any time of the year (generally folks that can travel off season or last minute and/or folks that can plan more than a year in advance have better success with exchanges)
Timeshare availability is not like hotel availability. All 52 weeks have been prepaid. So technically, timeshare resorts are always sold out. Owners have to compete to reserve the most desirable travel weeks. As the year gets under way, it's almost like the musical chairs game where owners scramble to reserve the most desirable weeks and some owners may be left standing without a reservation.
Read TUG's Timeshare 101 if you haven't - see
http://www.tug2.net/advice/TimeShare-101.htm
If you're interested in the hotel based timeshares take a look at this thread -
http://www.tugbbs.com/forums/showthread.php?t=34031
Don't rush into a decision. Take your time to decide what will best fits your vacation needs. Renting is definitely an option to consider.
Here's a link to the frequently mentioned rental websites -
http://www.tugbbs.com/forums/showthread.php?t=17673
Good Luck