At the risk of re-igniting the insoluble debate, I was prompted to finally put my 2 penneth in by hearing Pete Waterman on the radion the other day.
He is obviously used to this sort of thing and when asked about his "trainset" effortlessly explaind that as it was 390 feet long model railway was more appropriate.
The answer is you use the terms you are comfortable with, there can be no definitive descriptions if you look at the etymology.
miniature is something of reduced size
model is either a reduced scale copy or
an exemplary example (are the Swiss Railways a model railway?
So they are neither mutually exclusive nor equivalent
And when you get to narrow narrow gauge it's all purely relative after all:
The Severn Valley was part of the narrow gauge section of the GWR untill 1892
The South African Railways is Narrow gauge but it's not miniature
Of course in Victorian times the Festiniog was called a toy railway.
Which we probably consider this disparaging , but as the definition of a toy is a model or miniature representation, or a plaything; how could we in honesty disagree?
Clearly the forum needs a guidline to define what is relevant, and the 350mm threshold defined in law is important; but in general it's not worth trying to get pedantic about it since you can nearly always come up with a contrary argument.
Here is an example of a Moors Vally loco (Narrow gauge) next to the Britannia (model?) at Eastleigh; nice to see that a 7 1/4" loco that isn't dwarfrd by a 10 1/4