I think it comes with both actually, and gives you the option of which one to use when you install the OS. Which one you choose will depend on which OSes you want to be able to boot, as each bootloader has different capabilities. You would need to check the documentation for each bootloader to check which one is right for you. There are also tools which can help you to add/remove OSes from the bootloader without having to edit config files, which can be confusing (and possibly dangerous). One of those tools might even come with Ubuntu.
(Sorry I can't be more specific - it's been a while since I've used Ubuntu, and a loooooong time since I've manually edited a bootloader!)
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moto