May buy a seville sts, need some info on them!!!

I am thinking about buying a seville, first im not sure if I want the STS or SLS. What are the differences between the two? Are they reliable, cause I need to drive from cleveland to upstate NY once a month which is a 800 mile drive, plus driving to college every day. Do they get good gas mileage, and I did read something about a bose 4.0 system in them. What year did they start putting that in there? please help, cause i really need to decide soon.