What good does buying a climbing helmet do if you don’t wear it? Above all, pick a helmet that is comfortable and attractive in your eyes because if, on the other hand, you get an uncomfortable and unattractive helmet you will probably never wear it and it will do you no good.
Consider the type of climbing you’ll be doing. Are you at an increased risk of hitting your head or having something (e.g. rock, ice) fall onto your head? Or do you simply need a layer of cranium protection for the worst case scenario?
If your climbing helmet would be seeing lots of abuse then opt for a durable and more protective helmet. Otherwise, you might be able to get away with a lighter option.
Weight can be an important criteria for gear selection depending on the type of climbing you’re doing. Those seeking to minimize climbing pack weight for whatever reason should look for lightweight climbing helmets.
On the other hand, if you’re just spending a day at the crag and don’t need to pack as lightly as possible you can consider a heavier, more protective option. Just know that, generally, the lighter the helmet the less protective and durable it is.
Most helmets come in two adjustable sizes. Those two sizes span most head sizes but can fit weirdly towards the extremes (i.e. on people with small or large heads).