Well, if I can only choose one or the other... the fixed.
But actually it depends.
The first eyepiece I got was the Baader 8-24. With it I could have an idea of different focal lengths with my telescope, without having to invest too much.
I prefer all the other eyepieces to this Baader, but I have kept it because it is very useful when sharing views on the telescope with friends and people... there I prefer to use only that zoom.
The rest of eyepieces offer to me more quality and usability than the Baader, especially thinking that my main targets are doubles, planets and moon.
Regarding the other zoom I have, the 3-6, at the end I decided to have it instead of fixed focal lengths, because: it has good quality, it´s very handy and useful to change power with just a turn, and as those focal lengths are used less due to seeing, I prefer to have all-in-one for those powers, instead of having several pieces without so much time at the telescope.
At the end, if I had to choose between the zooms and the fixed focals I have, I would keep myself with the fixed ones, they´re so special for me.
Of course, I have not ever had a Leica zoom, that people talks so good about it, but I don´t think that its price matches the return you could get, at least with the telescope I have.
Regards,
Miguel L.