First, I have about 20 years of experience selling carpet cushion on the manufacturing level.

The L&P synthetic cushion will be fine except it won't feel like much. Part of the benefit of using a carpet cushion is that it provides some cushion when you walk. Synthetic fiber cushions are very dense and provide little comfort under foot. The 40oz. version that has been recommended to you is the most comfortable of all the fiber options, so you may find this acceptable.

There are a number of polyurethane cushions that will feel better when you walk on them that are treated to be mold and mildew resistant. Most major cushion manufacturers offer a bonded or prime polyurethane cushion with this feature.

I am not sure I agree with the comment that synthetic fiber will "allow the carpet to breathe." Not sure how that is possible.

Good luck,

Jeff

Thank you for the answer, Jeff
The reasoning behind using a synthetic pad vs a poly, is to not allow water to collect under the pad and to be able to get it out using a dehumidifier, if necessary. A softer pad would be nice, of course, but to me humidity is the #1 concern. While I don't expect to have water problems like we did in the past, I understand that water still comes up through the concrete due to hydrostatic pressure and if it has nowhere to go, it will just pool under the pad.