I windsurf in the Yorkshire Pennines (next to the highest Motorway in UK) and it gets pretty cold up here......
Needless to say, I have a 'few' wetsuits including shorties which I wear as an inner liner when I have been out on the water at -2C (yes, minus 2C)

The problem with heavy wetsuits is that although you stay warm, the (wet) weight is considerable and can be exhausting to carry around once wet.
The general rule that I use is, for ambient temperatures above 10C, the 3/2mm comes out. Less weight, less restrictive movement and less exhausting to wear.
If the temperature is less than 10C but above 4C then the 5/3mm O'Neill with chest heat shield gets worn.
Below 4C, I wear the 3/2mm shortie under the 5/3mm O'Neill which keeps me nice and toasty