i think i would rather run a VS than an XP, and an s1x more than an VS. maybe its just me and knowing how the cams run and drive.
the VS you can run on completely stock heads (inclusing valve springs). the XP and s1x you would need valve springs, but would run fine on stock heads (unported) as well. they are just bigger cams and the powerband will be higher up in the RPM range. they will make more overall power depending upon your setup, but it really REALLY depends on the tune and what youre intending on doing with the complementing parts.