In the "Something else" partitioning menu what partition table (MBR/GPT) does "New Partition Table..." use?
There is no default partition format for Linux. It can handle many partition formats.
For a Linux-only system, either use MBR or GPT will work fine. MBR is more common, but GPT has some advantages, including support for larger disks. The main issue with GPT is compatibility; not all OSes support it, and some can't boot from GPT on BIOS-based computers. For just a Linux system, I'd choose GPT. If you need to dual-boot with Windows, go with MBR.
That said, if you have a newer system, a very large hard drive, and you use UEFI I`d go with GPT.
The installer will use GPT automatically for disks over 2 TiB, if you choose the automatic option. It will create either the bios_grub or efi system partition depending on whether the machine supports UEFI or not.
So to answer your question, it depends on your system, hard drive size, and whether you have UEFI or not.
Master Boot Record (MBR) - Most IBM-based PC storage units use this partition table format. MBR is often referred to as the msdos partition table. The MBR can only support storage devices up to two terabytes. MBR supports the concept of logical and primary partitions. A storage unit with the MBR table can only have up to four primary partitions. Many users wanting to make a multiboot system with more than four Linux distros often have the problem of not being able to support more partitions. Remember, logical partitions cannot be made bootable. Multiboot systems must use a different partition table discussed later.Linux.org
GUID Partition Table (GPT) - Some IBM-based PC storage units have GPT, although it is usually because the user reformatted from MBR to GPT. However, most Intel-based Mac systems use GPT by default. The GPT partition table offers many improvements over MBR. GPT can support storage units up to over nine zettabytes. GPT is also the most highly recommended partition table for computers needing more than four operating systems on one hard-drive. For example, if a computer with a ten terabyte hard-disk is meant to be a multiboot system for seven different Linux distros, then GPT should be used. Most Unix and Unix-like operating systems can fully support GPT. However, most Windows systems cannot run on a GPT partition table. As for Mac systems, only the Intel-based ones can boot from GPT.Linux.org