I tried to stick to 2, anded up buying almost all and then relesing my own with a partner (which is nwo offered at White Ensign - Vent Set).
Very tricky. Some overlap, but they all have good stuff. Modelbrass really was only about the vent/flood holes, which are a must because it's impossible to cut out those bottom ones and make them look good and thin without beaking them. Eduard has fine detail and metal anti-slip deck bits, the vent set gives you accurate torpedo "cheek" pieces, templates, tensioners and turnbuckles and more, and the other sets have handwheels and various very good bits.
A very personal, biased view: Modelbrass, WEM 1 and WEM vents, Eduard.
Good luck and keep us posted!
AMP - Accurate Model Parts
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