the bumper plug things are the backup sensors...designed to warn you with a beep before you go crunch. As far as STS vs, SLS the STS is the sportier of the two cars (although neither is a slouch). STS has slightly more HP, and firmer suspension, and a shorter FDR, they were usually also better equipped.
As far as which one is better, they were designed for two slightly different buyers, drive them both and see which you like. Only because you asked about commuting, an SLS should (at least in theory) achive slightly higher (1~3) milage on the freeway, in town they will be about the same.
As far as which one is better, they were designed for two slightly different buyers, drive them both and see which you like. Only because you asked about commuting, an SLS should (at least in theory) achive slightly higher (1~3) milage on the freeway, in town they will be about the same.