The team that has qualified for the quarter-finals is the best team, whatever they have been through, what is certain is that the team that got here is the best of the opponents they have eliminated.
A slight difference is normal but not excessive so that any team will not be too favored or there will be no team that is easy to predict to qualify for the semifinals.
All the matches in the quarter-finals will be interesting and fierce and indeed the match between Manchester City vs Madrid is a little different in terms of prestige or tension because these two teams were previously favorites to win the UCL this season but have met before and that happened yesterday. This season makes the battle in the quarter-finals this season even more interesting.
Yeah I agree with you that is a fact but sometimes there are teams that are always lucky so they can reach the highest level, for example a team that is in an easy group with weak opponents well, sometimes we see moments like this in several tournaments. Arsenal is one of the teams that is in the easy group and is also playing against weaker teams but when they reach the quarter finals then we assume all opponents are equal. Next month Arsenal will have to fight against Bayern Munich there was no other choice they had to go all out.