I’ve got an El-Mariachi (for sale if anyone makes me a good enough offer) and had a Swift (now owned by another STW’er).
The Swift was the more ‘steel’ of the two, the Mariachi is stiffer, not always a bad thing, but it’s more ‘tough steel hardtail’ than ‘springy steel hardtail’. It’s still very much steel, but not as ‘steel’.
The Swift won’t take a suspension fork unless you track down one with a straight steerer. Not an issue of you plan to stay rigid though.
In terms of handling there’s not a huge ammount in it.
The Mariachi has more scope for adding pannier racks etc if that’s your thing.
1×10 cable routing was better on the swift, but mine was an old one not the new bolt on cable guide version and my Salsa is the SS version so cable guides are bodged.
Nothing to pick between the EBB and alternator dropouts. My alternators siezed (stripped the thread unscrewing it, or maybe somethign corroded electrolyticaly), but only cost £15 for a new plate. The EBB is easier to adjust as it can’t be out of alignment like dropouts and it doesn’t move when nipping up the bolts.
The Swift was the better looking.
The Swifts fork felt more compliant, but I didn’t quite ride the forks back to back. The Swift uses Reynolds blades, no idea what Salsa use but seeing as a lot of people use Reynolds blades it wouldn’t surprise me if they were the same although the Salsa forks do look like a larger diameter.
So in summary, the Swift was the better bike, but if I was going custom I’d:
Have the Swifts main triangles and fork.
A tapered headtube
EBB.