There are many things to consider when looking for a new case. What kind of construction are you looking for? How many bows holders do you need? How much storage do you need for your shoulder rest, extra strings, rosin and pencils? What other extras do you need?
Styrofoam saves weight, but it also sacrifices safety of the instrument. One good hit and the case is ruined, and you may be paying for costly crack repairs. Plywood, the standard case material is strong, yet heavy. Cello cases have been made from fiberglass and carbon fiber for some time, but now violin and viola cases are being made from these lightweight and very strong materials. As a rule of thumb, the lighter the case, and the better the construction, the more expensive it is.
How many bows do you have? One? Two? Four? Most violin and viola cases will come with either two or four places for bows and cello cases will come with two usually.
Storage is a big thing when it comes to cases. some cases come with a special pocket perfectly sized to fit a cake of rsosin. This keeps it from hitting other accessories and breaking. Some cases, like the tonareli fiberglass cases have just a small pouch and area for a shoulder rest. This minimalist approach saves weight and cuts down on the physical size of the case, whereas some, like the Ambassador line of cases, come with lots of pocktes to fit anything and everything you'll ever need at a gig.
More and more cases are coming equipped with hygometers to measure the humidity inside your case and small humidifiers to keep it at a constant. These small perks can keep your instrument safer when traveling and the changes in humidity that cause those annoying open seams or worse, detremental cracks.
