Wow...the 20oz is cheaper than the 12oz as a single!
IIRC the FIC looked like weak sauce from what we could glean from the PDS and MSDS in a previous thread...I would definitely go with the Concentrate Plus.
As a point of reference, the 12oz concentrate is something like $4.94 at my local WM every day. I have been buying Regane for the same price lately.
The FIC is ~$4.50 at my WM, so Amazon is not beating it by much.
If I bought a 20oz, I would probably use it as two doses and that looks like a good deal for $6.99 compared to two 12oz bottles at WM.