Tjyylli

I am confused what OS to use, between Debian 9 or Ubuntu LTS. Ubuntu sometime give me alert to reboot the system, but not with Debian 9.

Is Debian 9 more secure and stable than Ubuntu 18.04 LTS?

jcbermu's answer covered the gist. Let me expand around the edges and perhaps give you some context and a better understanding of the relationships.

Debian

The main audience for Debian is enterprise environments where the highest priority is that stuff reliably works. The second priority is that the IT support staff doesn't spend its time solving problems with the distro. That audience wants to install something rock solid, update it when/if necessary for any security patches, and generally not mess with it for a long time.

Debian's repository contains the key software that organizations need to get their work done. It typically isn't the latest, cutting edge versions, its time-tested versions that have been thoroughly debugged and work reliably.

Debian installations are managed by professionals who know their way around under the hood. They often find it faster to use terminal commands than GUI software. Polish and cool special effects and window decorations aren't all that important in that environment, and newbie-friendly tools aren't needed (although some are provided). Debian is optimized for a professional audience and is the best fit for people with some Linux proficiency.

Debian actually has only one "release", which is the Stable distro. The distro evolves a little at a time over time, it doesn't start from scratch for the next release. The developers have the current stable package base and a pool of candidate packages to consider for inclusion or replacement. Those packages go through a series of testing, evaluation, and integration stages in order to get into the next stable release. Debian makes the repositories for those stages available.

There are two development stages where the repository contains a complete collection of stuff that could actually be turned into a distro. Unstable is where most packages start if there's reason to expect that they won't introduce critical errors. Most of the focus there is just ensuring that the packages won't be major problems. They graduate from there to Testing. Not everything in Testing makes the final cut. The focus there is testing and ironing out the kinks of making everything work together.

Ubuntu

Ubuntu starts with Debian's Unstable repository, so it's really just narrowing down the universe of packages to look at. Ubuntu has its own testing, evaluation, and integration process to turn that into a distro. When that's ready for public consumption, it gets released as an interim release with a limited support life. Feedback from the public identifies bugs that weren't found in testing. After that public scrutiny, the refined interim releases get turned into the LTS (Long Term Support) release.

The Debian package base isn't the only components of Ubuntu. Ubuntu has a very different audience, which is everyday, non-expert users. Ubuntu adds utilities and polish to make the distro more user-friendly. Mint takes it a step farther by starting with Ubuntu's LTS release, adding it's own utilities and polish to make it even more usable by "non-techies".

If you are considering Debian, the closest you will get to its stability is Ubuntu's LTS release or Mint. Ubuntu's interim releases are considerably more buggy.

But Ubuntu's LTS release still isn't as stable as Debian Stable. There's a tradeoff. If you want it to be rock solid, it needs to stand the test of time, which means Debian's packages will be older. If you want everything very current, you can't take the same amount of time testing everything to perfection.

Debian's repository is also targeted at enterprise needs; their priority is not to offer every available package that any user may want for any purpose. Ubuntu tries to satisfy a wider audience. It has a broader range of packages in its own repository. It is also common for Ubuntu users to tap into third party repositories (PPAs). Ubuntu users are more likely to prioritize what they want, to do what they want to do, over doing a standard collection of business tasks in a super stable environment. Ironically, they're also the least prepared to deal with the problems they may create for themselves.

Security

Security is a different matter. Linux is secure by nature. If Ubuntu is less secure, it isn't the distro that's less secure, it's the distro's users. Ubuntu appeals to novice users, especially immigrants from Windows. Novice users often don't have a security focus, and often download anything from anywhere to try shiny items. They also tend to opt for convenience over security. So their bad practices can make their installation less secure, and they're more likely to do it on Ubuntu than Debian.

What's best for you?

Only you can determine what is best for your needs. If you have a little Linux familiarity, or aren't averse to doing a little research and "getting grease on your hands", your software needs are mainstream, and you would prefer to load and forget your OS, Debian could be a good fit. If you want access to a broad range of "consumer software" with recent features, want a very user-friendly OS, and have some patience to deal with occasional minor stuff that doesn't work right, Ubuntu LTS would probably be a better fit.

But keep in mind that you don't need to make a one-time, irrevocable decision. Pick the one that you think is more likely to be a better fit. If the shortcomings of that distro turn out to be bothersome, try the other one. There's no cost, installing Linux isn't outrageously time consuming, and you can do it in a way that has virtually no impact on your own files (how to do that has probably already been covered in other questions and can be found online, or could be the subject of a new question).

Comparing both is a very subjective issue.

The hard facts are that Debian tends to be more conservative. It means that the version of the packages included are being used and tested widely so they are less prone to bugs or errors (stability).

On the other side it means that you are working with older versions of the packages than in Ubuntu, so your system won't be using the latest features of the packages.