Pop-up goals type collapse together at multiple points throughout the goal (such as midway through one of the sidebars) whereas foldable goals typically just have the back triangle fold up so it is flat with the front of the frame.
Pop-up goals require a bit longer to set up and collapse, but break down into a smaller area. Foldable goals, on the other hand, are very easy to fold but will still be 6 ft by 6 ft since the front of the frame doesn’t collapse at all.
If you plan on taking your goal to different practice locations, you should buy a pop-up goal.
If you plan to keep your goal at your house or a different single location, a foldable goal may be the better option since they’re usually more durable and sturdy.