The two fetishes are quite different.
A laughing fetish is fairly specific in that it involves a very specific behavior. It is centered on hearing or possibly seeing a person laughing. Practitioners would probably have specific likes for volume, duration, breath pause, and so forth. It is primarily a passive fetish, in that it's observed. Though branches might swing on how the laughter was provoked.
Tickling covers a much wider range of possible behaviors. In that there is much more variety in what can be tickled, the spectrum of responses that range from vocal to physical and so forth. Laughter can be a subgroup within tickling that causes arousal, but it's just one of many.
Very different fetishes
Myriads