just found this review (1st on on google!) it explains the difference between the drives really well.
http://thessdreview.com/our-reviews/sata-3/adata-s510-sata-3-120gb-ssd-review/
ROFL - missing all the cut/paste changes from 511 to 510 looks bad.
Final stat shows that the M4 is about 10% faster.
Prices are interesting for the 510.