We'd like to monitor IPSLA on a Cisco Router.  I see there are two options in the alert drop down: PAVAIL and PDOWN.  Which would you recommend?

In this case it would be a better option to use the pdown metric. The reason for this is that Iris will alert as soon as the IPSLA link has dropped a certain percentage as set in the threshold. 

The pavail gives you an average of availability over the period of what is shown on the graph, so for alerting purposes, the pdown data source will be better to use.