What would be a better choice for a web server, Debian or Ubuntu?
I mainly use Python & PostgreSQL with Nginx and sometimes PHP & MySQL.
What are pros/cons of each?
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Advantages of Ubuntu:
Advantages of Debian:
Expanding on the answer given a bit to include some of the other common pain points that I find influence my decisions here:
Ubuntu isn't particularly committed to fixing problems even in their LTS releases. I've lost count of how many times I've ran into a problem and the only resolution was "fixed in [version + 1]", with no fix in earlier versions. Basically, you shouldn't expect backports of major bugs and security issues; from what I've seen the effective policy is that minor bugs are only fixed in the current release.
Ubuntu doesn't care about software freedom to the somewhat extreme level Debian does, which is one factor behind why many consider it easier to use. They're fine shipping things like binary images for drivers when that's the only solution; Debian is not. For example, to get Debian to work on my laptop, I have to explicitly turn on the non-free repository and add the appropriate packages during or after install. Hardware is more likely to just work out of the box, or with a much less difficult configuration step, on Ubuntu.
Related to that, if you plan to deploy servers on a cloud computing platform, as well as some other closed source virtual platforms, you'll normally have better luck with Ubuntu. On Amazon EC2 for example, it's trivial to run on EC2. Meanwhile, you can't even get an integrated copy of their EC2 tools for Debian, due to an unsolvable licensing issue. Work on Debian EC2 is accordingly much less popular, and there are similar issues with other virtual machines, too. Basically, if your vendor isn't releasing their whole stack as open-source, the odds that they will accept Debian's strict definition of free software for parts that must go into the OS are low.
Stepping back for a second, I also like to use the same OS on my desktop as on my server; makes life easier if I can focus more time on a common platform. I'm finding it hard to use Ubuntu as that platform lately because they are so aggressively breaking their desktop with unstable code. The Debian desktop is boring, but it works and gets out of my way. Those are good qualities for a server OS, too, if you do ever want to run a GUI tool on it.