What to consider when buying ice screws.

First of all and most importantly is the length of your ice screw. You need to know your terrain because depending on the depth / quality of ice during your adventure the length of your screws will vary too. 

In general you can divide ice screws up into three categories, short, medium and long screws.

Screws ranging from 10-13cm can be placed in thinner ice without hitting the rock behind it. It is always good to have a few of those short ones especially in the early season because the ice will be thinner. 

16cm - 17cm Ice screws make up the majority of a classic ice climbing kit. If correctly placed in good, hard ice they are really strong.

The longer screws up to 22cm are good when additional security is needed, for example if the ice on the surface doesn't see, strong enough. 

After having considered the most important aspect of your ice screw purchase you now have to see which one is easiest and most comfortable to place, considering you'll be wearing thick gloves the whole time through. 

Some ice screws have a knob or handle that unfolds from the hanger.

Finally the last thing to keep in mind with ice screws is that you better store them in some sort of reinforced bag so that the teeth don't damage any of your other material. 

Here is a cool article by La Sportiva on placing ice screws.