The S and XS were first, and the early ones sometimes developed a crack in the control housing where the brass pin for the upper cam lock goes thru. That was fixed as warranty work.(The backup XS I bought several years ago was cracked. I still had to pay to ship to them, but I don't remember about return shipping.) Then came the EXII, SE, SE Pro, E-trac. Roughly, I believe the S & XS came out around 2000, EX II 2004-2005. The SE & SE Pro 2006-2007 and Etrac Sept of 2008. Three of us hunt together quite a lot. One uses an XS, another an EX II, the me an Etrac. For our personal monthly silver count competition, it is normally the XS and Erac with the most and the EXII third. The XS and Erac go back and forth, but XS is probably ahead. (This info can be misleading. It doesn't show which machine is best, it shows that the machines all are good at finding silver and which operator is better with their particular machine, and also, who hunts the most times). All great machines. I presently use my original XS as my backup, and sold my other backup XS to my XS hunting partner for his backup. I do not know anything about the SE or SE Pro, so no input there.