If Leverkusen and Stuttgart can reach the semifinals of the DFB Pokal, I still believe that the one who will be superior is Leverkusen. Because so far Leverkusen has not been defeated in the Bundesliga so it is very possible that Leverkusen can also become champions in the DFB Pokal, moreover everyone has also seen the quality of the players these two teams have so the confidence of most people will remain in Leverkusen even though Stuttgart also not a bad team this season, including in the DFB Pokal.
Going by the present form and performance so far, its very true Leverkusen appears superior and even the the DFB pokal they are still unbeaten just as they are across Europe and I don't think they will be willing to bring an end to that record firstly in the pokal as it will appear disgracing to them after facing bigger teams and doing much better against bigger team in bigger leagues.
Stuttgart is much above average and wouldn't be seen as a weak team playing against Leverkusen, at the bundesliga they have performed really well but this seems like their performance in the DBF pokal is still on an average hence the superiority of Leverkusen will most likely supersede the ability of Stuttgart. There are every chances Leverkusen should win but then if uncertainties of football activities play out, it's possible Stuttgart can aswell turn out the winners but then Leverkusen is the favorite and has got the better chance.