I have a new IG pool that is 14500 gallons with the IC20. We just had the pool guy put in whatever he recommended. So far we keep it at about 60%, I think we may even be able to go down to 40%. It does have a 100% mode where it shocks the pool for 24 hours, but taking some advice from these forums, I am shocking with bleach to save on wear and tear on the unit.

I guess if you can afford it, go with the bigger unit unless someone can come up with a better reason not to.