AMM (anti matter missile) are very useful if you want to have a high militray ranking with a low maintenance. A high military ranking avoid AI demanding tribute to you for the reason that you aren't enough worthy to exist.
Which ship should I build - Dreadnougt and Deathknight cost the same to maintain
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Compared to a Deathknight, a Dreadnougth have less hit points (-10), more attack (+2), more defense (+3), more speed (+1), less range (-1) and is cheaper (-200).
So if you have the choice, a Dreadnought seems to be a better choice.