Hi emurphy,
Both of the foundations you mentioned or any of the options in the foundation post here would work fine for a memory foam mattress without using any plywood on top of the slats. The 3" guideline is for a mattress that has latex in the bottom layer because latex is much more elastic than other foams but for mattresses that use polyfoam for the support core (such as most memory foam mattresses or a latex/polyfoam hybrid) then larger gaps are fine. I would avoid the use of plywood when possible so that there is better ventilation between the mattress and the foundation (see post #10 here).
And congratulations on your new mattress 
Phoenix