I wouldn't consider normal paprika the best substitute for cayenne pepper but you could get away with it if nothing else was at hand. Paprika is very mild and it's mainly more a colourant for a meal than a taste enhancer, however hot paprika would be an adequate substitute for cayenne pepper as it is very hot. If the focal point of the dish/recipe is the peppery heat/hotness, then I'd go with the hot paprika, black pepper or even chilli powder as a substitute for cayenne.

Paprika is a little too mild compared with cayenne, it really depends on what you are using it for, if it's just a garnish then yes you could get away with it, but as an ingredient I would use chili powder or even hot sauce or you could use crushed peppers. usually if it is an ingredient you want the kick.