  Are rewards or penalties more effective in influencing user behavior? This
work compares the effectiveness of subsidies and tolls in incentivizing user
behavior in congestion games. The predominantly studied method of influencing
user behavior in network routing problems is to institute taxes which alter
users' observed costs in a manner that causes their self-interested choices to
more closely align with a system-level objective. Another conceivable method to
accomplish the same goal is to subsidize the users' actions that are preferable
from a system-level perspective. We show that, when users behave similarly and
predictably, subsidies offer superior performance guarantees to tolls under
similar budgetary constraints; however, in the presence of unknown player
heterogeneity, subsidies fail to offer the same robustness as tolls.
