-or and -er noun endings

[Dilbertian]

Why is it an incinerator, not an incinerater? Gardener, not gardenor? Apart from pronunciation, what is the difference between an executor and an executer? Both execute instructions.

I've often wondered this.

Closest I can get is that when it's -or, it's often a word with Latin origin (auditor, lector, spectator...), and Latin uses -or (eg caveat emptor).

Apart from those, English usually prefers -er.

Interesting question.

I have no idea whether there is any rationale for the differences but I rather like the typically English inconsistencies.

Barrister, but solicitor
Lawyer, but doctor
Writer, but author
Painter, but decorator and sculptor
Miller, butcher and baker, but tailor
Teacher, but professor
Manager, but auditor
Driver, but conductor

There must be masses more.

Adviser is much more common than advisor these days, but for some reason I prefer the latter.