Glow Plugs come in different grades. They are usually indicated as a number or as hot, cool or medium rating. For most standard 2 stroke applications a medium to hot plug will be suitable, ie an Enya 3 or OS6 / OS8 . For a high reving engine, such as a performance model car or a ducted fan, then a cool glow plug should be used.
The hotter the plug the thinner the element and the cooler the plug the thicker the element.
A good indication is that if you have a performance hot engine you should buy cold plugs
Hot plugs promote better idling and acceleration. If your engine runs rough or accelerates sluggishly, a hotter plug will help.
Cold plugs produce more power and may improve performance if your engine runs hot. The downside is rougher idling and more difficulty in tuning.