Kind of boils down to this poll, eh?
But definitely BSB for me, because in comparison to SES it has an upper deck, neutral sails, so it can be converted into any faction by swapping flags, a much better stern design, and no silly rotating canons. SES is of course a proper three-master and the big sail on the second mast makes it look a lot larger and more menacing. If I'd have both the SES would be my pirate ship, whereas the BSB would be converted into an imperial ship.
Luckily I just managed to buy a 6285 BSB