When does a 7mm beat a 5mm wetsuit?

Below about 17 °C, on repetitive days where a 5 mm stops recovering between dives, and any time your logbook says "cold by minute 30." The 7 mm costs shoulder mobility and noticeably more lead, which is why nobody wears one at 24 °C for style. Between bands, a hooded vest over the 5 mm is the versatile bridge. Bands are guidance; your chill is the spec.