I know that technically both pronunciations are supposed to be "correct", but I've yet to meet anyone that believes that. Everyone thinks that one or the other is right.
Me? I think that "Seltic" is correct. In English and in Gaelic (I've read), a "C" before an "E" is pronounced "S". Besides, it's the Boston "Seltics", not the "Keltics".
What say you? (Or what kay you?)