I cannot base this on any hard facts but my own, insignificant experience.
In the UK all the men called Mark I came across so far (including my husband ;-)) are spelled with a "k" at the end. Sprinkled in are a few called Marcus - usually then spelled with a "c" in the middle.
In Germany it is the complete other way round (would explain why the majority of mails or invitations we get from Germans are still adressed to a non-existent "Marc" in my life). Here Marcs are usually belonging to the "c" fraction while the, more common, Markus has a "k". Of course there are exceptions to the rule but I found this interesting...If there are any linguists reading this who might have an explanation for this, please let me know.