Should I use tasksel, tasks in APT or install regular metapackages?

Tasksel or plain Apt?

In the past I've always used tasksel to install so-called "tasks". It seems, however, that this has been integrated in APT:

apt-cache dumpavail | grep ^Task
[...] snip
Task: lamp-server
[...]

and that I can install such "tasks" by appending a caret (^) to it, like this:

sudo apt-get install lamp-server^

Is this equivalent to the following?

sudo tasksel install lamp-server

And which is preferred?

Metapackages

Additionally, how do metapackages come into play here? Some tasks seem not to be a metapackage while others are:

apt-cache show lamp-server
N: Unable to locate package lamp-server

apt-cache show kubuntu-desktop
Package: kubuntu-desktop
[...]
Task: kubuntu-desktop

So, using the last example, what is the difference between the following three possible ways to install?

• sudo apt-get install kubuntu-desktop
• sudo apt-get install kubuntu-desktop^
• sudo tasksel install kubuntu-desktop

In short: There is a difference between tasksel and apt-get installation process of tasks.

Looking at your example:

The apt-get way

sudo apt-get install 'lamp-server^' will evaluate to do the following:

1. Search the cache (package list files) for all "Task:" fields and get all with "lamp-server".

2. Install those packages the standard way:

   sudo apt-get install package1 package2...
   

The tasksel way

sudo tasksel install lamp-server will look for a task called "lamp-server" in one of its configuration files under /usr/share/tasksel/**/*.desc:

1. Search the configuration files for a "Task:" field named "lamp-server".
2. Check if all prerequisites are available ("Key:" field) and remember them if not installed.
3. Choose method to select packages ("Packages:" field). If this ist "list" simply install following packages by name.
4. Read all following package names and pass it to previously selected method to generate package names.
5. Look for /usr/lib/tasksel/info/lamp-server.preinst. If it exists execute this script.

6. Install packages with apt-get using following command:

   debconf-apt-progress -- apt-get -q -y -o APT::Install-Recommends=true -o APT::Get::AutomaticRemove=true install package1 package2 ...
   

7. Look for /usr/lib/tasksel/info/lamp-server.postinst. If it exists execute this script.

Conclusion

tasksel is more powerful in processing and selecting tasks. It can execute extra scripts before/after installation/removal of tasks. And the biggest benefit: You can modify tasks and create new ones very easily. It is not possible to edit an official package list file without drawbacks (valid signature).

Back to your first question:
In your special case both commands are almost equivalent (supposed you have enabled both APT::Install-Recommends and APT::Get::AutomaticRemove). Only difference is the extended package state of mysql-server and apache2 dependencies (set to "manually installed" with plain apt-get).

And what about metapackages?

If you don't make use of tasksel's features like selecting tasks (especially helpful at installation) and executing extra commands before and after some task blocks then a task is very similar to a metapackage. The difference is: A task is not registered as a package in APT cache.
So if you uninstall one task-dependency other task-deps aren't marked as auto-installed because they were explicitely installed. If you uninstall a metapackage the dependencies are removed with autoremove because their extended package state is "auto installed" (if not installed manually).
Note: All distributed tasks in Debian install a metapackage named task-TASKNAME.

Giving your example:

1. sudo apt-get install kubuntu-desktop
   • Will install the metapackage kubuntu-desktop. Dependencies are "auto installed".
2. sudo apt-get install kubuntu-desktop^
   • Will select all packages tagged with task "kubuntu-desktop". That are all dependencies of metapackage kubuntu-desktop. They all are marked as "manually installed".
3. sudo tasksel install kubuntu-desktop
   • Make sure X is installed before installing packages of task.
   • Will install the metapackage kubuntu-desktop. Dependencies are "auto installed".