No, it's the other way round. Ignore Portals is what you want to use under ideal conditions. On DM-Gothic, with Ignore Portals I get 2.29:1, while Portals Cut All is 2.36:1. That's because Ignore Portals means that they are treated pretty much like standard geometry - the BSP optimiser can put them in the tree nicely. Portals Cut All also cut the tree down, but on the positive side it ensures that portals work right. In short, if the map is created properly, that is, all zone portals were added the last, then you are supposed to go with Ignore Portals. If they weren't, then you're forced to go towards Portals Cut All, because otherwise zones will start leaking, which in turn will make it more difficult to optimise the map. And that's why it's a slider, not a radio button, you're supposed to experiment with it to see where zones start leaking and put the slider just where they don't yet leak.