I've looked up previous threads on this name and lots of Mumsnetters seem to prefer the Caspar spelling. I generally think it makes more sense to go with the most popular spelling of a name and as Casper is currently number 346 and Caspar is number 606 I thought Casper would be the sensible option. Why do people prefer Caspar?
Also Kacper is number 172 but I'm guessing this is maybe a Polish variant?