Choosing between __traits(compiles, { ... } ) and is(typeof( { ... } ))

[Nordlöw via Digitalmars-d-learn]

When is

    __traits(compiles, { ... } )
preferred over

    is(typeof( { ... } ))

and vice versa when writing stuff like

    enum isCopyable(S) = is(typeof( { S foo = S.init; S copy = 
foo; } ));

?

Further, are there cases where the two idioms aren't exchangable?