Hard to compare exactly, the spesh forefoot is wider, but the spesh upper uses a stiffer imitation leather and doesn’t adapt to your feet as well, which rubs if not a perfect fit.
The sidis narrow to a point too much requiring a size larger for me, but overall much more comfortable (for me). The sidis are useless for walking on, the sole is made from slick plastic I have nearly died several times just going through the front door with my bike.
I would say that the spesh uppers come straight up from the insole, and expect you foot to fit, the sidi uppers wrap around your foot and adapt to your foot. Does that maker sense? its hard to describe.