The SG31G2 uses a really dated elanpower PSU, the SG33G5 PSU is much quieter and has higher rated rails. The G33 chipset is also quite an improvement over the G31 chipset. Overall the SG33G5 is a better SFF than the SG31G2.
Looks wise in the flesh the new xpc cases are more classy imo, also better laid out inside to.