I'd prefer the spelling "Christian". That would get the pronounciation you want. Christen looks like Kristen to me.I'm pretty neutral between Christian or Tristan. I don't really favor one of the other.
I agreeI also recommend going with Christian rather than Tristan. People will constantly pronounce Christen like Kristen.Out of Christian and Tristan, I think I prefer Tristan. Neither name is really my style though.