Keep in mind that the aider step will sag down when you step into it, so a 20" step-to-step can easily become a 22-23" step-to-step. Just keep that in mind when sizing. Also, if you're on the fence about 2 or 3 steps, etc. just get the extra step. If you don't need it, then just don't use it. Not hurting anybody to have an extra step hanging there unused, but can sure be nice if/when you want to use it.
A good way to try out aiders to see what works for you is to buy the cheaper amsteel and fuel-line aiders which are sold many places. Try a few with different step distances and different numbers of steps and you'll quickly learn what you like. Sell the others, or keep them around as spares.