The Foundations have less mids and are a little bigger sounding IME. I like them on a G&L which has tons of mids baked in. This is not to say that are hallow. I have the NPS.
This is what I got back from John H. at SIT with respect to Foundations:
The Foundations were designed with tonal balance and string flexibility in mind. They aim to sound less like an "angry piano" and more like a good old fashioned bass guitar. The Foundation Nickel variety is slightly brighter than our Power Wound series, and the Foundation Stainless will be slightly less bright than the Power Steels. The Foundations also have new core-to-wrap ratios resulting in a string that is also easier to bend. For what you're describing, we would personally recommend trying the regular Foundation Nickel variety.