I have had bad luck with the shiny black rubber/plastic ones not fitting in the first place, or falling off of the tip when I am not looking.
If the string locks it on, it will be fine. That means either a leather one, which is great in dry climes, or the plastic Saunders one. In wet places, the leather will stay wet for a long time, which is bad for the tip. Even the rubber ones should be removed after hunting in the wet, to allow any water collected to dry out, rather than sitting on the limb tip.
The rubber ones that I use do not get locked onto the tip by the string, but they seem to hold fast and do the job.
Killdeer