Typically the Studio models are a bit higher end and have a little bit more high performance internals. If you plan on buying from dell's website, you can configure your own. Sometimes they have deals and you could configure and Inspiron to something comparable to a Studio for less money, but it all depends.
Yes, and Dell Sudios are also thinner and lighter. Usually Dell Studios have a better graphics card. If you want something that is good for internet and lightweight, easy tasks, you should pick Dell Inspiron. Otherwise, if you play some games and things like that you should pick Dell Studio.