I have owned both, I liked the newer version in .357 for the smaller dimensions and lighter weight, but I found no difference in quality, accuracy, or how the sights were regulated between Vaquero and New Vaquero models.
I started CAS shooting two handed but within a year or two switched to "duelist" which is one hand only on the gun. I ditched SASS and only shot NCOWS the last 6 years I was in it. NCOWS is a smaller group, but the rules and structure of matches was more interesting to me. Everybody in that branch of the sport shot only duelist, and while two handed was legal, they were quick to tell you that the only people in the old west who shot a SAA with two hands were women or children.