The first and most affordable case is a gig bag. This is a soft sided bag, often padded and zippered, that is most commonly used to transport your instrument. You want to choose a padded gig bag over an unpadded one. It does not offer any protection against humidity and the instrument can still get damaged (dented, dinged or broken!) while in the gig bag. It does provide a handy way of carrying your instrument as backpack style straps are often included. If you have a very expensive instrument, a gig bag is not recommended for storage, only for travel and even then only if you can be very careful and avoid collisions.
Next is a semi-hard shell case. This type of case can include shock-absorbent foam or plastic molding to protect the instrument from damage and usually features a fabric covering with zippered closures, such as the Guardian Featherweight Cases. The semi-hard cases does not protect against humidity, but does usually prove resistant to dents, dings and actual breakage of the instrument. This can be a good option for longer term storage of your instrument, and for travel where you are in control of your instrument's handling. They are a good lightweight option.
Hardshell cases provide very good protection and they can range from affordable to high end. One example of high end hardshell cases is the Ameritage line. This line provides humidity controls and temperture controls! There are arch top and flattop cases available. Archtop cases provide superior protection but a flattop can offer limited protection against crushing. Hardshell cases often feature plush interior lining to cushion your instrument. One of the downsides is that hardshell cases are often heavy - which can be a hassle to carry around. This is an excellent long term storage solution for your instrument, to ensure mint condition.
How do you know what case is the best fit for your instrument? It is highly suggested that you purchase the manufacturer's recommended case for your instrument if possible. The manufacturer will often have a recommended case but, if that is not available and your instrument is a common shape, then other brands or especially custom after-market cases (such as the Ameritage line) will be a good choice. Ameritage even has an online measuring tool if you are interested in purchasing one of their cases and want to be sure to get the proper fit. Be sure that the retailer is an authorized dealer of Ameritage cases.
A few other important points:
* One of the most important thing about cases is that you want it to fit your instrument snugly!
* If you purchase a hardshell case, ensure that the instrument is not jiggling around inside the case.
* If you are unsure, take your instrument down to a local music store to test out different fits of the cases.
* It is recommended to store your instrument in a place that is not exposed to extreme temperatures. This is especially important if you have a wooden instrument as the wood can warp and change the tone and sound!
* Keep your instrument stored in a cool, dry area
* Store your instrument where it is safe from falling objects or falling down.