Yes, that is another sentence. There is a difference: σκεπτικός means someone who doubts a fact/statement etc while ο σκεπτικιστης is a follower of a branch of philosophy by that name which as its name says involves "disbelieving". http://www.thefreedictionary.com/Skeptics So, both, of course, are correct.
Jaye, this distinction exists in English as well. Someone who is sceptical [adjective] of something is disbelieving/doubtful of it. Someone who is a skeptic [noun] is a follower of Skepticism. (I have heard people use "skepticist" to mean the latter as well, but I think it's redundant and, for what it's worth, my spell-checker doesn't like it either.)