For a number of years we had both - two different time shares, one with II, the other with RCI. I REALLY liked II's last minute (60 days, I believe) trades, where you could trade anything for anything (week for week, but upgrade for free). We used to regularly trade into Orlando (which has a great over-abundance of units) in that window and would get 2 and 3 bedroom units for our limited kitchen 1BR trade. I knew I could always wait till the end and have no problem getting a trade, except possibly in the Christmas and Easter weeks.
What I didn't like about II was that my limited kitchen and floating weeks had almost no trading power except that Orlando area.
our RCI week had similar trading problems, but now with RCI points we can combine several year's worth to get a better trade. MUCH better from our perspective. And what really sold us on RCI is the ability to trade into Disney Vacation Club.
One other plan I really like is under the VRI umbrella. With that we could use a poor trade and upgrade to a better unit by paying an "out of season" fee. Made our trade much more valuable.