If you look at the results of the matches played by Chelsea in the Premier League and Borussia Dortmund in the Bundesliga, the two teams are currently in an unstable form, because they are experiencing a decline and are unable to keep up with the match. If you look at the current standings in their respective leagues, it is likely that next season they will have difficulty qualifying for the Champions League, Borussia Dortmund are in the top six of the table and Chelsea are in 10th place, which means that both teams are in an unstable position and little worse than the previous season.
Borussia Dortmund in the Bundesliga still have hopes of still qualifying for next season in comparison to Chelsea it will be difficult to get a place in position 4 this is far from a possibility unless Graham Potter makes significant changes in the league then it is still a possibility but judging from the matches yet stable then it will still be tough as well.
This is indistinguishable in my opinion the Premier League is tougher than the Bundesliga.
But it seems that Borussia Dortmund is slightly better than Chelsea, even though the Champions League cannot be seen only from the performance of their respective leagues. It is still quite difficult to predict who will win, before seeing the first match which will take place at the Signal Iduna Park Stadium in Germany.
Playing in the Champions League is also always different in appearance maybe also for Chelsea but we don't know what will happen later with Borussia Dortmund who deserves to win, but right now it's still difficult to predict even though there is still a chance to qualify and you can say it's a balanced team if you look at the current performance in each of them.