Single or Double Critical Ejector?

Will you get a higher discharge pressure for an ejector designed for single critical or double critical flow?

Single critical is where the motive gas flow is choked (sonic) in the nozzle but the entrained gas is not choked (subsonic). For double critical both are choked. To compare realistically, the motive and entrained flow rates, temperatures and pressures must be identical.

The short answer: sometimes yes, there is some benefit - up to a point. As can be seen in the plots below of some examples, a reduction in entrained gas Mach No to 0.7 can increase the compression ratio.