As an organist, it's JSB every time. Mozart wrote very little for the King of Instruments and his Fantasia in F minor for mechanical organ is a beast if your hands are small. My son's French horn teacher would not allow his pupils to play pieces by Mozart in audition situations but I have had to learn a few accompaniments to his horn concertos for my pupils. They are enjoyable but I'd rather play the large-scale Bach organ works.
FF, as an oboeist, which would you prefer? Bach's Chorale Prelude 'Nun Komm der Heiden Heiland' sounds fabulous as an oboe solo.