The descision tree for choosing between one and the other would be pretty complex. The needs of the user, your layout, whether you are using IWP and whether or not you will print from the layout are all factors that would have to be considered.
One key difference is that a list view can be dynamically sorted and searched to pull up different groups of records via criteria specified by the user sorted in orders specified by the user.
On the other hand, Portals allow you to use a small portion of your total layout space for the list freeing up a "form view" design option for what you do with the rest of your layout.
Portals is never going to be as fast as showing related records in List View or Table View.
I use Portals to show very little related data and give user a arrow to go to those related records in List View. Then it is up to user if they see enough information or need more from the List View. Make sure to have good navigation going back and forward between related records and records you where going from.