Your best option will depend on what exchanges you want. RCI points isn't really going to get you many more options, particularly since you can now combine points from multiple weeks (or leftover points).
II would get you access to resorts that are only affiliated with them, but there are other ways to access those resorts, such as through some of the smaller independant exchange companies - some of them deal in pretty much all resorts affiliated with both RCI and II. Why pay to purchase another week, plus another membership fee, and maintenance on another week, when you could trade what you already own through one of the other companies? And for some locations, one of the smaller companies may be a specialist, which would increase your chance of getting those exchanges.