Recently built the XI SPORT and raced it in VTA, great car and you cant beat the price.
Ordered a S2 and should have it soon, not to replace the XI but to compare the two.
On paper the S2 looks to be more adjustable out of the box:
-XI has one location for upper link on bulkhead cover, S2 looks like it has 3 locations.
-XI has 2mm arm spacing(wheelbase) adjustment front and back, S2 looks like it has more.
basicly pick a color, both are great cars for under $120