The only major diffrence i see is, the P4SGX-MX will accept both DDR and SD-RAM sticks. While the other takes DDR only.
SiS and VIA are companys the design chipsets.
Both Motherboards where in the 50-55$ range at newegg. The P4V533-MX got a really low rating (doesnt say who, or how many people reviewed it), the other board got no reviews (from newegg).