In a .40 P-Series Ruger, I would recommend a P944 over the P91. Specifically, a later/"second generation" P944, with the cam lock (or is it cam block?) recoil system, identifiable by a black polymer filler piece on the left side of the frame; these guns are significantly more durable over time compared to their earlier swinging-link P91 and earlier-production (also swinging-link) P944 brethren. The swinging-link works both well and durably in 9mm P85/P89/P90/P94 (9mm) guns, but the higher pressure/more quick pressure spiking characteristics of the .40 cartridge can cause some long-term challenges.
Otherwise, the P94/P944 is also more desirable in my opinion due to some other engineering/component changes Ruger made to these guns to provide for a smoother triggerpull and an overall more efficient system.
Best, Jon