Should I use MBR or GPT when I intend to use both Ubuntu and Windows?

Either MBR or GPT would work.

MBR is limited to 4 partitions... but due to a bug in Microsoft's Windows updaters, major Windows updates tend to wipe out MBR Linux partitions and GRUB (on single-drive installations). MBR tends to be used on older BIOS computers.

GPT doesn't have the partition limitation of MBR, and it looks like your Windows disk is probably set up as GPT, and it's good to have them setup the same way. GPT tends to be used on newer UEFI computers. Disks that are 2TB or larger must use GPT.

So I'd vote GPT.

Don't use the Windows Disk Management application to format/init the SSD disk that's going to be used for Ubuntu. Only use Disk Management for work in Windows, and with NTFS partitions. Use gparted to work with Ubuntu and ext4 partitions. Boot to a Ubuntu Live DVD/USB, then use gparted to lay down a fresh GPT partition table (this will wipe the drive), and then use the Ubuntu Installer.

Before using the Samsung SSD, I'd check that the firmware is current. Use the Samsung Magician software that can be found at https://www.samsung.com/semiconductor/minisite/ssd/download/tools/. This tool runs under Windows.

Microsoft recommends using UEFI for Windows which requires GPT. It is also a more modern system. https://docs.microsoft.com/en-us/windows-hardware/manufacture/desktop/windows-and-gpt-faq

Ubuntu's bootloader GRUB will only load Windows if Ubuntu and Windows are both either BIOS or both UEFI.

It is best therefor to initialize both Windows and Ubuntu as GPT.

Unplug the Windows SSD when installing Ubuntu. When done plug it back in, boot Ubuntu and run sudo update-grub to put Windows on the GRUB boot menu.