I am using Ubuntu 8.04 and I want to try the upcoming release of Debian and I wanted to know which repositories is better Ubuntu or Debian in some topics:
1- Which one have the new package updates up to date?
2- Which one have more packages?
3- Which one have the newest packages?

And I have a last question, can I put Debian repositories on Ubuntu and vise versa?

The situation is not that simple.
Ubuntu is based on Debian - with some in-house development.

Changes made in-house are usually presented upstream, where they may or may not be accepted. So you can expect some Ubuntu packages to differ from Debian (or other) ones. Debian and Ubuntu repositories are not, in general, compatible.

So - with that in mind:
1 - It generally averages out. The trick is to consider the particular areas you are interested in keeping up with.

2 - Debian has more packages.

3 - Debian testing will have the latest, otherwise it averages out.

In other words, you need to be specific.

Ubuntu uses their own heavily altered packages. If you want to use Ubuntu use their repos, if you want to use Debian install Debian.

If I'm not mistaken Ubuntu is based on a snapshot of Debian sid (unstable) Trying to grab packages from the Debian repositories would probably give you OLDER packages than what is in the Ubuntu repositories and will at some point probably break your Ubuntu system.

I agree:
Use Ubunutu repositories for Ubuntu.
Use Debian repositories for Debian.

In case it needs said again...

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