I prefer
Conor. I'm American but I still see
Calum as kind of... laddish, I guess? I definitely pronounce it with two distinct syllables.
They might get misspelled occasionally but it would only take one correction. It's not like they're weird spellings that nobody's ever seen. I used to know a Konner.
I don't like
Connor much but I love
Conor. It's much more friendly and approachable!
Callum and
Calum give me the same vibe, although
Calum looks more like it could be a noun instead of a name. (not that it doesn't look namey, just seeing
Callum and
Calum next to each other makes me think
Calum looks a bit noun-y in comparison)