I think what we've failed to consider is that price may simply not have been an issue in the least. For example, if I want the fastest car, I'm going to shell out around $515k for a 250mph screamer. On the other hand, I could spend $150k for something that does 200mph. That's a roughly 350% price difference, whereas the performance difference is only 25%. However, if I want THE fastest, I'm gonna be shelling out half a million.
Similarly, if the most important criteria was cabin size and comfort, then the Merlin must be chosen. The S-92 may be cheaper and better-performing in its factory trim, but it will never have the cabin size of the Merlin.
Whether or not cabin size was the first priority is up for debate, but if the Marines simply wanted THE biggest, the correct choice was made.