Espion is similar to PhantomJS in that they're both headless browsers that you can inject JavaScript into.
Espion comes with a lot more. First there is the infrastructure: processing power, storage, connectivity and IP addresses that you don't have to provision, set up or manage. Second, Espion includes the features that surround extracting data from a site such as job scheduling, data quality monitoring, online debugging and problem resolution and data delivery.
PhantomJS is perfectly viable, but if you need the features I highlighted and use it, you'll have to build a lot yourself to get the job done.
