I’ve got both!
I’d be happy with either, only bought the 2nd as I wanted a large over a medium (which means there’s a medium swift frame in the shed £250 collected from Wokingham or posted at cost).
The El Mariachi is stiffer, especialy the forks (or maybe the headtube?), the Swift feels more ‘steel’ all round. They both ride very similar, just subtley different, there’s very little to seperate them in my usage (~20mile rides, almost entirely on the Gorick XC courses so lots of tight, flowing, rooty, often muddy singletrack)
I’ve never had an issue with the EBB, but do make a point of setting it up/forewards. Never creaked or slackened.
Had one alternator dropout bolt’s thread spontaneously corrode, not sure what caused it, must have been a galvanic circuit formed somewhere. They’re more faff than an EBB (6 bolts, all different sizes and they need to be adjusted parralel). The EBB you could adjust in a muddy field blindfold, alternators need daylight and a workstand makes like easier.
Also the SS el mariachi can’t take gears (except 1×10 with a new droopout and a full outer), the swift takes either setup with really cleverly hidden cable guides.
Given a choice I’d probably get a Swift El Mariachi whichever you like the look of. I’d like to sawap back to a swift and try it but my new forks have a tapered steerer.