An O ring chain will generally last a lot longer than a non O ring chain. It also means your sprockets will last longer, (unless you are pedantic about replacing your worn chain) because worn chains accelerate sprocket wear. The down side is O ring chains gobble up a bit of horse power, which is why they are now somewhat out of favour with the 250x4 stroke MX guys.
They are dearer, but last longer so may not be dearer in the long run.