seems like everyone has been talking about a old SJ. the new 08-10 models have a redesigned hull for more race style hookup and turn, and they have been doing well in the races. its still surfrideable so keep that in mind. the SXR is a race boat, period. it is not a nible playful feeling ski, its more like a sitdown in how it sticks to the water. surf riding is extra difficult because it just loves to bury the nose down the face. i can assue your not a big guy considering a 650 pulled you well so your not going to want the extra wide and bulky feeling boat that the sxr is. reliability of the srx and superjet is debateable, stuff will break, but they are both very good models of watercraft. its the old kawis that have the issues.
so i think you will be happier with a superjet.
forget about flatwater, get in the surf and stay there!