I started with the clarinet at a very early age and then the piano at ten. I learned classical theory on the piano which ended up being a mixed blessing when I studied jazz theory on the guitar.

It would drive my teacher crazy that I’d think keyboard first at the expense of the guitar fingerboard. Sort of a filter that made it more difficult to automatically play what I was thinking/feeling at the moment and when composing.

Can’t say my folks did it “wrong” but in terms of the OP’s question; I’d say to go with whichever instrument the person most naturally gravitates towards. Piano is a more versatile and complete composing tool but the guitar ain’t no slouch and it’s far more portable.