Is spiders building webs on plants an example of mutualism or commensalism?
You're thinking too broadly and rigidly about these concepts -- both commensalism and mutualism are types of interspecific interactions. 

Commensalism is when one species in a given interaction is benefited while the other is neither benefited nor harmed. In a mutualistic interaction, both species benefit from their interaction with each other. 



Like any interaction between two organisms, these interactions can vary between individuals. Although some species pairs will consistently undergo the same type of interaction (e.g., shark and prey -- predation), some groups of species may vary in their paired interactions. This variation may result from two different scenarios:


Variation in interaction type between different species within a taxonomic group 


E.g., one type of spider catches insects that don't harm the plant -- commensalism -- while another spider species catches insects that would harm the plant -- mutualism. In other words, not all spider-plant interactions are the same, and the interaction type may be dependent on the species involved.

Variation even from the same individual in different circumstances. 


E.g., A spider catches an insect that doesn't eat the plant it's growing on (commensalism) but at another time it catches an herbivorous insect that does eat the plant the spider has built its web on (a mutualistic behavior). In this scenario, the interaction type between the same two species (even the same individuals) can vary based on circumstances. 



Put differently, commensalism, mutualism, etc. are not behavioral traits, but interactions, and so are dependent on not only the species involved but the circumstances as well.

Note: To further complicate things, each given interaction can be qualified in various ways. For example, from Wiley ELS:


  Positive interactions can be trophic and nontrophic, can act directly or indirectly (mediated by a third species) and can be symmetric (species have equal effects) or asymmetric (species have unequal effects).