We just bought the On-The-Go system. The double size, and are happy so far. The smaller size would have probably fit in the wet bay but I will have to Regen the larger one less often so I guess there are trade offs.
 One thing I found out by setting the unit outside. If it is in the sun it acts like a water heater LOL!! and takes a while to run through the hot water it creates. Between it,, a whole house filter and the hose you have a bunch of water.
 We have the snap on mesh type wheel covers and we now set the softener behind it if in sunny areas and that helps a bunch.
Just thought I would bring it up because it's something I hadn't thought about until standing in the shower wondering why the heck I couldn't get any cooler water