Toro ss4200 or sw4200????

I have recently bought a toro ss4200 mower and I mow a couple yards and 3 of them a some pretty good slopes and when I mow with the ss4200 the mower just wants to slide or spin. I was jw if I would be better off with the sw4200 or if the more I get used to this mower the better it will get??????????