I was told that using a 'z' is old-fashioned, and using it or an 's' is optional - just be consistent. Only recently have I noticed a trend to condemn it as 'American', and if anything is sure to bring out a rather nasty, old-fashioned snob in some English people, it's being 'American'. Personally I cringe at this and quite happily use the 'z' form.
On the subject of anti-American snobbery, one of my friends whom I have known for over 30 years was adopted by a very old-fashioned English couple and (sure enough) often had a disparaging word about anything American. When he was about 20, his natural father made contact with him ... from America! After things had settled down a bit and he had met his 'new' family, I put it to him that, you know what, you are basically an American now. Foolish to pretend otherwise. He's changed his view quite a bit since then :)