Vetting, training and whether anyone stays.
Traditional contingent labour fills a seat. An agency sources from a local pool, adds a markup and places someone who is often a generalist with light vetting and little onboarding into your systems.
Connext recruits against your specification, trains the person into your processes and tools, and keeps them: retention is measured quarterly rather than hoped for. The comparison turns on time-to-productivity and turnover, which is where the markup on contingent labour usually gets spent twice.
Contingent labour places people quickly, and when a spike is genuinely short that speed is worth paying for. Choose Connext when the work outlasts the spike and turnover starts costing more than the markup ever did.
Follow an engagement from the day you sign it to the day you judge it. At each milestone the two models do something different and at the last one, the BPO wins.

Figures as published on the source comparison page.