Portfolio diversity is the default, and it's the one we recommend.
Portfolio uses the Portfolio Diversifier, which builds your whole set of lineups as one complementary bet. It picks lineups that win in different ways, weighs them by contest and entry fee, and solves for the best combination across every entry at once. We backtested it against Min Uniques and global exposure caps, and it produced stronger ROI with less downside.
Uniques forces each lineup to differ from the others by a set number of players. That makes lineups look different without making sure they depend on different outcomes, so it's a weaker tool for the same job. Use it only if you specifically want a fixed number of unique players between lineups.
On Ultimate, Portfolio Plus runs full Contest Sims for each contest inside the diversification, so the final portfolio is judged against the real field and payouts. On Starter, Portfolio uses a generic contest sim for all of your portfolio data.