what exactally is it that attracted you to the car? is it looks alone? If so stick with the sv6. Its got a little punch and averaged great economy when you dont drive it hard, and most of the bugs have been worked out of the tuning and engine.
The SS is an awesome package, and when driven respectably only uses about 3lt/100 more than the sv6 on long trips. The Auto trans that it mounted behind both the SV6 and SS is a 5sp Active select auto. If you are a real driver then go for the manual 6speed. The 5speed, has taller ratios which make it a little more sluggish off the line and the active select/tiptronic reaction is slow in both models and the torque convertor seems to lag when cold.
It all just depends on what you can afford, and what you can justify.