Simo, I just went through the same debacle as yourself when wondering about the Seville's.
The SLS and STS are the same car, different trim levels. The main differences have already been stated: tighter suspension, different gear ratios.
The big thing for me though is that the STS is a higher trim level, and it means that some stuff that's optional on the SLS is standard on the STS- so you get more.
Check out
http://www.edmunds.com for a good overview of the differences, and also for pricing- they have this True Market Value thing that hits the mark for the used market. I picked my Seville up on eBay for $19,500 to my door; Edmunds said I should of paid $21k. (-;
All in all, shoot for getting an STS if you can do it.