I recommend that you spend some time going through the online availability of the alternative systems. What I found is that each has strengths in supply and availability in different geographic areas. Plus, you should do this multiple times over a period of time, as one snapshot in time does not show the picture for the entire year, because deposits come and go. Focus on reviewing the areas you may want to trade into, or else it will take A LOT of your time.
We use II, TPI, Platinum Interchange, DAE, and SFX in addition to RCI. But we won't always have units on deposit with them at a given time. Since I do not know where you like to travel, or the quality of resorts/amenities that you want, it is hard to recommend one over another. Certain resorts make large bulk deposits with these companies based on their agreements, so as you go through the online availability, you may notice that some resorts show up a lot in some trading companies but not in others. Plus, some of these will tend to have lower quality resorts for their bulk deposits, but you may find some gems sprinkled in among those in TPI and Platinum for example.
If you open a free account with any of these (except II with its mandatory fee) you will be able to view the inventory. TPI and DAE allow you to see their availability without an account. Some more desirable units are only available to you further ahead of the check in date if you upgrade to pay the exchange company for higher status such as gold level, etc. Another thing to work the system is to wait until they come up with an offer to give you a second or third trade with a deposit you make (these extra trades are usually limited to 45-90 days in advance of the check in date, so you must be flexible to take advantage of those).
Be sure to look at their upgrade fees if you lockoff. Some are pretty high if you deposit a studio and then want a 2 or 3 bedroom to trade into. Some of these companies bury this info in obscure locations on their websites.
We typically plan well ahead of time, so I plan deposits of our multiple units to these different companies based on that. For example, DAE has the best availability and inventory for New Zealand, so we made a couple of deposits with DAE last year to plan for our upcoming 2017 trip there.
One last thing - if you do make a trade with one of these smaller exchange companies, be sure to double check that your resort knows who they are and confirm that they got your trade information. I was surprised how many people I worked with on this at our Marriott (which we deposited) hadn't heard of TPI or Platinum when I first went through this.