In this matchup, I think Paul/Toomey would win, since Paul is a fairly strong candidate and might be able to barely defeat Clinton. A best case scenario for Paul would be a weak economy and a mediocre campaign by Clinton:
Rand Paul/Pat Toomey - 288 electoral votesHillary Clinton/Tim Kaine - 250 electoral votesHillary's best case scenario is to can tar Paul as an extremist, hold Obama's coalition together, and hope that Toomey never catches fire as a viable running mate.
Hillary Clinton/Tim Kaine - 323 electoral votesRand Paul/Pat Toomey - 215 electoral votes