SY has better DSE results than SFXC. Admission to SY is highly competitive as many Band 1a students in Kwun Tong Net will choose it at Discretionary Stage.
SFXC on the other hand is not that competitive as most of the YMT Band 1a student will choose WYK and another Band 1a school from other school nets at Discretionary Stage. Therefore, Band 1b students can still have a chance to get an offer from SFXC.
Both schools are strong in Maths / Science but their languages results in DSE are just mediocre for a Band 1 school.