Actually we can't compare their performance when playing against other teams, because it's not a worthy opponent, the situation will be different if both teams fight each other and something unexpected can happen. Apart from that, the home team factor can also have an impact on their performance so they have a greater chance of winning. I'm not saying Barcelona is worse because they are at the top of the table, but I think Real Madrid has a better squad in all aspects.
The meeting between Real Madrid and Barcelona will always provide a fairly tense match and every coach from the team must have been very aware that it was an opponent that really had to be defeated with their respective best tactics. So I also won't judge both of them as good or bad when playing against other weaker teams this season, because a much more accurate assessment to say is when the two meet in the La Liga match this season because they will definitely use all their abilities to win.