I can give you a 100%, iron-clad, definite answer of ... it depends.
In most cases, a portal is used to display (and sometimes edit) records that are related to the current parent record. For this, it suits very well. It's considerably harder to do that in List View (although it can be done).
However, sorting a portal is quite a bit trickier than List View. In List View, you have all the native FileMaker tools and really don't have to do much else (of course, you can if you want). Portals, however, have a single sort order built into their definition. To bypass that requires some extra work; there are multiple techniques out there for accomplishing this, but none of them is particularly simple to accomplish. And many of them involve a significant performance hit.
You also have the ability, in List View, for your users to perform a native FileMaker Find to filter out whatever records they choose. Portal filtering works fine, but you have to program it into the system.
Of course, List View does have a few downsides. The aforementioned link back to the parent record is one. Another is the inability to do anything like a sidebar (such as what you see on many web sites) without some extra trickery (like using a second window). And, of course, if you want to be able to control what records users see, then a portal is easier.
So I guess I can't really give you a "this one is better" answer. Maybe if you let us know exactly what your application is, we might can steer you a little more accurately.