Hard to compare due to the way they play/played as very different players both class & able to influence/change a game. Still not sure I'd say either was World Class as there's really only a very small number who I would ever put into that bracket.
Bit late to this one but Gazza was always the one for me that I never saw justify his hype. The distance between how pundits raved about him and how I saw him (occasionally good for a few minutes a game) makes him as overrated as anyone IMO.
I think Gazza was one player who could have genuinely been top class as had all the skills & attributes but unfortunately too many demons & so neither reached anywhere near his potential & destroyed his own career.
Following mention of Rooney, Scholes and Falcao, as far as the Manchester United of today are concerned I think they're littered with overrated players such as Ashley "Every Rs Favourite Guano Gobbler" Young, Chris "How Do You Control The Ball" Smalling, Rubber Johnny Evans, Phil Jones and Adnan Januzaj. There are more, which probably explains why they're no longer challenging for the PL title these days.
Most overrated player for me is Yaya Toure. Must be the most immobile player around. The main reason why Manchester City have failed in Europe time and time again. Their over reliance on him in the Premier League causing them to lose when faced with better opposition. Regarding Steven Gerrard Ciarai, not sure if it's your memory mate but he is everything but overrated imo. Physically, technically and mentally superior to most of the players he's been up against makes him world class to me. He has always come through when his (club)team has needed him the most, a player for the big occasions, something that can't even be said about C. Ronaldo.
Based purely on this season alone, surely Falcao from MU, at 300K per week must be the biggest rip orf known to mankind. Then again, we have SWP & Adel both on 60K a week and neither of them play at all .............. but l better not mention them.