Ubuntu tries a lot of different things, and the environment swings around wildly. Sometimes you'll see a howto that has different steps for each of the past four or five releases (6mo apart). Debian is much more considered in it's changes, and is better suited for server environments. The Debian philosophy is quite clear, and you never have to keep an eye out for encroaching adware or phone-home stuff in the core systems. Debian also does less 'magic stuff' for you; for example, if you mistype a command, Debian will just 'not found' it, whereas Ubuntu will suggest-sell you a command... which takes time for the results to be parsed and presented.
On the desktop, Debian has quite a few more rough edges, and you wouldn't really recommend it for a newbie.
Regarding redhat being out of fashion, part of that is that Ubuntu was a big drawcard to bring devs to linux because they focused on polishing the desktop, so the debian family got an influx of users. Personally, I don't like redhat tooling as I find them full of gotchas and their output is usually full of chaff and hard to parse. That may be just a personal taste issue; I'm sure plenty of redhat admins find debian tooling weird and odd.
On the desktop, Debian has quite a few more rough edges, and you wouldn't really recommend it for a newbie.
Regarding redhat being out of fashion, part of that is that Ubuntu was a big drawcard to bring devs to linux because they focused on polishing the desktop, so the debian family got an influx of users. Personally, I don't like redhat tooling as I find them full of gotchas and their output is usually full of chaff and hard to parse. That may be just a personal taste issue; I'm sure plenty of redhat admins find debian tooling weird and odd.