Hi Felipe,
On Traditional Web, you can use both.
Personally I would use the Modal from OutSystems UI which has more options (see image below) and I find it easier to use:

With the given options you can effortlessly create a sidebar or a toaster message, just adjusting position.
It is also more modern, for instance, in the popup, you have a loading more message that is visible for a second while that is not the case with a modal and it also does not have animations out of the box as the modal.
Hope it helps.
Regards,
João