I belong to both. 90% of my trading is RCI.
It may change when my kids grow up and we retire and have more flexibility on when and where we go. RCI has way more local stuff in my area.
My view, generally, is if you have a highly desirable location and week and want to trade into a nice place, II may give you an advantage. RCI is more flexible, and people with less desirable intervals can use the flexibility (to combine and extend deposits in weeks or to have a points account) to trade into units they could not typically get in II.
My prime season, gold crown, (non-fancy brand) Williamsburg VA 4-BR lock-off resort can be traded in either II or RCI. It has more trading value trading into DVC in II than what it had in RCI. (On average, it would get about 1.5 or less trades per year into DVC using RCI. Trading in II it will get (match for) 2 trades into any DVC unit that appears in general inventory. (DVC ratios could change tomorrow, but that has held for the almost 1.5 years DVC has been back in II.)
But my actual preference right now is RCI because it has more where I want (can go) AND it gets more value trading down to less valuable weeks since you are not forced into 1:1 trades.
Note: I generalize a fair amount here in ways that I feel leave the right impression. But there are a lot of exceptions and nuances when it comes to timeshare trading.
In my view II's fees are slightly more reasonable than RCI's -- but they have a duopoly. The lack of competition keeps fees higher AND removes incentive for them to be innovative and more efficient.