I never watched Tufnell bowl during what I consider to be his peak apart from highlights, so I'm going from what I've read from various people, and statistics.
Basically, I think that Tufnell had the potential to be the best spinner we've had since covered pitches, and at his peak he in fact was. Up until the 1992 World Cup, he only averaged above 30 in one series, and that a debut series in Australia. And it's not as if he didn't cause an impact there - he had a catch dropped off a hat-trick ball, and generally had a **** time with umpiring decisions (umpire called McConnal IIRC, anyway he sounds like a bit of a **** to me). This run includes bowling England to victory several times. None of this run was played on traditionally turning wickets - no matches in Asia IIRC.
Anyway, after the 1992 World Cup was when the personal problems started, and he was never as consistent afterwards. I did watch him several times after this (watching him bowl out the Aussies in 1997 was what made me a bit interested in cricket in the first place) and he always looked a handful on the turning wicket, and looked innocuous on non-turning wickets, much like Giles.
I've stated my opinion on Giles in the other thread, and I do rate him quite highly for a finger-spinner, but I'm not convinced he was any better than Tufnell was 1992-onwards, and Tufnell pre-1992 was (I think) a considerably better bowler.
EDIT: I've just realized I've made two long (for me) and reasonably thought-out posts in CC in twenty minutes - unheard of.