I just added the little one 2 weeks ago. Since I wasn't sure about what it really could accomplish, I just got the small one.
It does make the water flow more consistent, and will give us a flush and quick hand wash without the pump kicking on. The initial "blast" of pressured water does wonders on the flush.
Next year, I may get the 2 gallon one if I can find a deal. I see these things ranging from $150-$200 a pop.
In our setup, I did want to quiet the thing down, so I remounted it and wrapped it in bed foam. Cut the sound down by half.