I would have picked Sirotkin over Kubica as well for one simple reason. Kubica is too old. Kubica may be a great driver but he's coming to the end of his career as a top level driver, in a top level sport like F1. The young lads coming through are showing they're able to keep up with the fast pace of F1 better than the older guys IMO.
Most sports these days are for young lads, the support they get from the team and trainers and all the staff that helps in the background just erodes any advantage a lot of experience can give. Which means their faster reactions win out.
Sirotkin is young, at the start of his career and just has more potential than Kubica. Put this on top of the fact Kubica is carrying an injury that calls into question his ability to complete a season and the fact he hasn't been racing means it's an easy decision to make. Even before looking at money incentives.