The PBY was a 35,000-pound plane with edit (Thanks, Milosh!): 2,500 mile range. The Do-18 was a 22,000-pound plane with 2,100 mile range.
The advantage is the Catalina here, but perhaps only slightly.
They built 3,300 Catalinas, including Canadian units. They built 170 DO-18s. There is no comparison there, the Catalina is the winner, rather overwhelmingly. 3,300 even largely inferior planes are going to contribute more than 170 superior planes. One-on-one, they might have been close. Collectively, they aren't. Perhaps you meant one-on-one?
Add this; they are still flying Catalinas. No WWII German flying boats are still flying with the excpetion of maybe one Do-24 that was rebuilt as a possible turboprop new-production unit some time in the last 20 years. It did not make production Here is a link to a video of it.
https://www.facebook.com/View: https://www.facebook.com/JukinVideo/videos/957551014333169/
About halfway through, note the altogether unique landing maneuver. Now THAT's something you surely don't see every day! This was and IS a very nice flying boat. That spin-out was an anomaly caused no doubt by a pilot who put it down while it still had flying speed. That is, he didn't hold off until it stalled into the water. Notice when he touched down that he touched way forward, more or less about at the 1/3-chord line. When that happens, interesting things follow. Rather, he landed when it still had enough speed to lift a bit when the nose came up.
I'd chalk that up to simple pilot error, and I'd bet it never happened again!
First flying boat toe loop I ever saw, and it wasn't even in an ice rink!