If I want to collect bandwidth usage data from edge interfaces for analysis , what is the recommended way to get at that information? Via the REST API? If so, are there any recommendations on how frequently to poll?
Given that the Edge appliances are virtual machines, you can monitor the performance, in this case the volume of traffic, on a per-adapter basis by way of the vCenter Web Client. vRealize Operations Manager can take that to the next level, by way of dashboards, trending and the ability to aggregate multiple metrics into supermetrics.
As larsonm mentioned, you can just get the raw stats for the edge VM's interfaces via vCenter using the performance management API or similar. What VMware would recommend, though, is that you look at vRealize Operations Manager with the NSX management pack. It auto-figures this stuff out for you.