Twin Tank vs Single Tank

Hello,
We just moved into our new house and we need to install a water softner.

I called in a specialist and he gave me options for a Clack water softener system, Twin Tank (1.5cu ft each) or a single tank water softner (2.0 cu.ft). We are a family of 5 and we have a basement that we may put up for rent. He conviced me to go for the twin system which I later agreed to.
Few days later he sent me a quote for the Twin Tank, and I realized he quoted me for a Clack water softener Twin Tank (1cu ft ) and not the Twin Tank (1.5cu ft) which he initailly mentioned.

Now he is trying to convince me to go for the Twin Tank (1cu ft) and not (1.5cu ft)

I am acomplete novice when it comes to water softners, and this whole experence has made me more confused. Should I just go for the initial Twin Tank (1.5cu ft) that we agreed on? or even the Single 2.0cft