Guys being taxed as "non-resident" are paying 24.75% (if I remember correctly) on everything.
Guys "tax-resident" pay normal Spanish tax on salary & hourly rate, the 1300€ is not taxed. This gives a pretty good balance between gross/net for the FO's, but I suspect the Capt's salary would attract higher tax rates, the FO's get away with 20.1% on salary + 4?/ 5? % social. . . I would guess a large part of Capt salary would be at (say) 40ish%, so , if not resident , non-resident tax would be the way to go I would hazard.