Installing a distro alongside an existing one

Most instalation guides ask if you want to install that distro alongside an existing one. I presume that the existing partition is resized to make room for the new one.

Assumning that I have backed up the existing distro. Does it make a difference whether I let the installation do that resizing task, or would it be better for me to prepare the way with gparted and resize the old one and then create a new partition?

Depends on your goals. If you want it quick and easy, but without having power over any decisions, then you may use the guided way. Otherwise, if you want to have power over the situation, partition it yourself.
Personally, I always used the manual way.

So no difference in the quality of the operations performed? I remember when you said that formatting NTFS from Linux was problematic. If I do it manually, there will be a reboot before I install anything. Does that affect the results?

That is correct.

If it is already existing, there won’t be any NTFS created.

I myself prefer having a dedicated storage medium for each OS. Then, customizing my partitions, as I want them to be, precisely.

There’s no difference in the quality of the installation. The installer will show you its intended division of the hard drive to accommodate both OS. Unless you see some glaring problem just accept it and the installer takes care of the rest. I have done this often as I have done a lot of distro hopping. I also have done the partitioning myself as I have learned more in my Linux journey.