This probably won't be the most popular opinion here, but I prefer a four-person party over six. This is mostly because I don't want to manage that many characters - both from a mechanical perspective (dealing with gear and inventory, picking spells, etc.) and from a story perspective. I want to focus on a characters. In my ideal world, I wouldn't actually have direct control over the companions - I could interact with them and form a plan together, but how they execute that plan (and whether they even stick to the plan) would be entirely up to them and their personality. I like playing from the perspective of my character - not an entire party.

In addition, I rather enjoy having a party that isn't well-rounded. If the tools we have available are restricted, we have to get more creative in the ways that we solve problems - this is a good thing (assuming the game is structured in a way that is sufficiently flexible). If I can have access to everything in one party, that removes some of the more interesting choices I have to make. I'm all for burning bridges and jettisoning companions after act 1 - this dramatically improves replayability.