A distro mainly distinguishes itself from other distros only in the kernel version it uses and the set of programs and applications it distributes to the user by default. Also some distros handle the user management differently. Some don't allow you to login as 'root', but they add your normal users into a wheel / sudo group if the user is marked as an admin, so this user can do almost allthings a root can.

This of course doesn't hinder you to configurate the installed linux distro to your needs. They are basically all the same. The distro only defines what is on board when you freshly install your system, to reduce work for you.

I like arch, because it is small and doesn't come with lots of applications installed, despite all necessary drivers for so you can build up on that.

Of course you can do pen-testing with linux. Although, it might complicate things as you have to install the programs by yourself. ubuntu might not provide these apps in the official package manager, and maybe there are no binaries available. that means you would have to install from a different repository or build that program by yourself. It complicates things, but it doesn't make it 'impossible'. Building tools is often not that complicated either. Often, even a noob like me can do that ($ ./configure; $ make; $ make install). But can be very difficult sometimes too, if build-tools and dependencies are not installed on your system. CentOS for instance provides group installs. That means you can yum install a group of development tools, that simplyfies things a little bit.

While Kali as pen-testing distro provides these tools by default, ubuntu is aimed at 'consumers' with cloud services and everything a modern consumer os provides, though you can install all the programs from kali to your ubuntu installation and do whatever pen-testers do.

For a beginner, I would rather suggest not to use kali as your 'daily-os', since it opens doors for potential hackers. As a noob, like I am one, (but with life experience), I would suggest to install one pen-testing program at a time, maybe on a separate pen-testing partition (linux - linux dual boot) and extend as your knowledge grows.