In the studio, it's irrelevant. Whoever they pick will definitely have the technical ability, and also won't have any kind of impact on the creative process. They could replace Portnoy with a drum machine and it wouldn't affect the album at all. Live on the other hand could be different. Portnoy was the one person in the band who actually has some kind of stage presence (Petrucci kind of does I suppose, Rudess just kind of looks awkward, Myung literally avoids the spotlight, LaBrie leaves the stage every time somebody plays a fill or solo and goes backstage to do whatever the hell he does). I'd rather have somebody who has 95% of the technical ability and can be the showman the band needs then somebody who is effectively a fleshy robot.
Totally off topic but didn't the band say a couple of times that they'd be going back to the Evening With format for the next tour