I suppose that's it, it's a matter of one's own priorities - you can do the big main line in P4 but it will take a lot longer, if that sort of scene is your main interest and you don't expect to live to 150 maybe you will trade off some appearance for the chance of getting it made - EM or 00. Personally I would prefer to trade the other way, if I could only have the look of finer flanges and flangeways etc by settling for a much simpler prototype that's fine.
As you'll see on the left my current interest is Irish 00n3, that's using basically EM standards, but only because I can't find an 8mm P4 spoked wheel (anyone know of one?), if I could I'd accept I might only produce 2 or 3 locos rather than 5 or 6 and go for P4.
Just a personal choice of what's most important to you.