I think I am a longer in the tooth than most modellers contributing to RMW..
I joined the EM gauge society in 1966, and the S4 society in the mid seventies, so Ihave a foot in both camps. However my layout is to EM gauge and works. I do make my own locos and of the 27 Midland and North Eastern, 12 are scratch built, and some of the kit built ones sport scratch built frames, or chassis as some call it. All my tank locos are compensated as are some of the tender engines, and the latter have pick ups on loco and tender, so building working engines to 18.83mm gauge does not present any visible problems.
If I were starting again, it would be EM, since it satisfies me.
When I used to go to shows I saw some superb layouts in both EM and S4. I also saw some dreadful ones. So EM, S4, whatever floats your boat, and how comfortable you are modelling in either.
The thing is do it.
Derek