ΔE 7.4, close: similar, with a visible difference side by side. Middle Brown is a little darker, more saturated than Rust.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Middle Brown is a little darker, more saturated than Rust.
The same difference applies the other way round: Rust is a little lighter, more muted.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Rust on the colour wheel See Middle Brown on the colour wheel
All matches: Rust equivalents · Middle Brown equivalents