I've installed all three and driven them quite a bit. IMO the VS is a waste unless you add the rockers on top and for the added expense you aren't gaining much over either the XP or S1X.
When I started building I did Gen V and S1X on a car that already had the headers. Tossed a N* and LQ4 in front of it and wow. Great combo. Awesome power and it made a reliable, enjoyable daily driver. Since then I've done the same basic setup with XP's and S1X's for guys in the area. The S1X and XP are close enough and similar enough that going with either will net you the same smile. Most guys won't notice which has better low end or which has better mid/high w/o a dyno.
