I tend to focus heavily on skills when choosing helpers. A squad with 80k power with a 6* and a couple 5* usually fares worse than another helper I have with 75k and Cattleya + Edelweiss + Camellia as this squad can simply destroy the boss fight in a single turn when their skills activate. Also AoE skills can be very helpful to kill the boss adds and maximize squad uptime.

I agree with Tenhou that speed is relative, although high speed (not 700+ though) tend to have more uses due to being able to avoid small pest clutter. On def x hp I used to prioritize hp heavily, but with the team getting stronger and have less trouble with the content we get I am starting to see def in a new light, mostly because I think that small pests do a %hp damage (I might be wrong), and they're by far the greatest threat as I've limited options to avoid that damage depending on the map.