I sent my son to both of them and I prefer RSM by far. Kumon to me was rote learning - lots of very similar problems. My son did not last even the first semester.

My son then attended RSM from first grade. RSM instruction started from problems like "there are 3 birds on a branch. 1 bird leaves. how many birds are now on the branch" and progressed onward. By grade 7 he is learning logarithms and, at a very basic introductory level, abstract concepts such as function

(by function I mean the real definition of a function, not the easy f(x) = 2 x + 1 - https://en.wikipedia.org/wiki/Function_(mathematics))

As a victim of Kumon I am still traumatized from Vieta's formula to establish relationships between the coefficients of a polynomial and sums and products of its roots.

I stuck with it for 6 months during self study and now I see it everywhere in the world.