Souchaku had die cast armor in most of it's parts.However SH Figuarts had more details and better paint job.Furthermore,Souchaku armor parts tend to fall off easily over time but SH is just one piece thus this problem is removed.
Souchaku is fun because you get to wear armor for your "body" but SH Figuarts provides something that every fan loves,articulation.
Another difference would be Souchaku had lesser exclusives compared to SH Figuarts which practically list everything cool into net exclusives