Honestly, I've really never seen the word being spelled as "skeptical". I am aware of the possibility of it being spelled in that way, but I've never actually seen it. I always spell it as "scetpical" and that's how I find it in books, newspapers, etc.
P.S. I'm talking about the American English, not the British one. But to be perfectly honest, besides few examples (like colour, realise, grey, honour...), I don't really know which one is British and which one is American. When I'm writing, if my spellchecker doesn't mark a word as wrongly spelled, I consider the spelling to be correct XD So, my answer would be, use whichever you want. You are definitely going to be understood by everyone :) That's what I do, anyway.