Ronaldo spent six years under Fergie at Manchester United after arriving from Sporting Lisbon in 2003 and has worked under Mourinho for the past couple of seasons since 'The Special One' was installed as Real coach in 2010.
“It's not fair to say, it's like comparing a Ferrari with a Porsche,” Ronaldo said in The Sun newspaper.
“Both are very good, very experienced and both like to win - and I mean to win everything. That's why they have both been champions many, many times.”
Ronaldo faces former team United for the first time since leaving for Madrid in 2009 in a world record £80 million deal when the two clubs meet in the Champions League.
“Manchester was my home and still is in my heart. I love it," said Ronaldo. "Because when people treat you very well you never forget that.
“And I will never forget United, the people who work there and the supporters. For me it will be a pleasure, a real pleasure.
“Of course I know there will be a lot of attention on me but for me there is no pressure, no pressure at all.
“It is only a game, just a football match against my old club. I'm not going to war, I'm going home - and I am really looking forward to it.
“On the day of the draw I was out training and I had this strong feeling that we were going to get Manchester United.
“And when the draw was made I was very happy. It's a tough match of course - Madrid and United are the two biggest clubs in the world and it's a real 50-50. It could go either way.”