I have the SL Grand which is the more expensive one. I think the SL studio is the cheaper one. If I remember correctly, the big difference is in the feel of the hammer action keys. I think the Grand has wooden keys and a different hammer action with a more acoustic piano feel. While the Studio is a bit quicker, plastic keys (?), still hammer action, but maybe better suited for synth stuff, too. adminm@'s suggestion is probably very good too.
As with guitars, I feel like midrange gear is solid and should be useful for anyone. So, many of the suggestions would likely serve you well. It's more about what one does with the instrument than the instrument itself. For instance, Domi, from Domi and JD Beck, was using a cheapo m audio midi controller (which I really disliked the feel of) for a few years, and playing better than most of the world. But she's a freak of nature.