From a safety standpoint, I am thankful that all of ours (#340 & #41) open outward. Inward opening doors would increase the chance of catching the edge of the door while trying to run out the front of the cab. So it makes sense that the reigning regulatory authority would dictate such a thing.
As to the advantage of inward-opening doors, I doubt appearance was an issue. More likely it was because outward opening doors are likely to open parallel to the boiler, and they get in the way of accessing anything that sits behind it. Also, if a locomotive is in motion, you have to step past the open door before you can grab the stanchion (unless you have long arms like me.)
I'd lay odds outward door preference is a safety issue and inward preference is a convenience issue.