I've not studied at either, but am active in aerospace engineering professionally, both in industry and academia.
I'd say that Delft is highly regarded as one of the world's leading universities in aerospace engineering. Most league tables would put in the in the top 20 in the world for the subject.
Until I read your post on the other hand, I didn't know that the University of Liege offered the subject! A quick check shows that it doesn't seem to appear in any of the usual international league tables for engineering.
Interpret that as you will, and perhaps it puts the relative ease of getting into each into perspective.