Dave,
Both will work very much the same. Although balsa is a very soft porous wood it feels harder than the leather surface and many people like it for the finest of pastes like the 0.25. I use it for that as well and even for the 0.5 at times.

The all leather is still my most popular choice though and works well for all grits. Just opinion but if choosing a 4 sided strop and wanting to use balsa I would use it only on the 0.25 or 0.5 sides leaving leather for the more coarse pastes.

Tony