At your weight, the small difference in weight between those bikes is not going to be something you will be able to notice. The 07 SL has difference geometry than the 08 bikes, so check into that (08 bikes have taller head tubes).

Either the SL or SL2 are plenty stiff. The SL2 Pro is a decent package as well... but you can get 08 SL framesets for not a ton of coin these days. At $2200 retail it is a pretty good value for high end race bikes.