I've actually owned both of these guitars at the same time.
I found the tone of the Gibson marginally preferable, but the playability of the Martin much more suited to me. Whether this was due to the fretboard radius, or fret spacing, or perhaps some other factor, I can't really say. They are actually quite different animals, ideally you have to play them and choose your favourite.
All I can say is that ultimately I would rather play the Martin. Just my opinion, hope it is helpful to you.
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Martin D28 Std.
Gibson J45 Std.
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