My 2 pieces is of 3 mm, and my 1 piece is of 7mm.
I use my 3mm farmer john for windsurfing (need to move my arms, but I am not always in water) and sailing (same condition). Only if I am told to flip my sail to the water and I need to jump into water (for practice), or of course when accident really happen, I might be in water and I'll need to restore the sail and get back to the boat. If I really need to put something extra on while sailing, I will use a wind breaker on the top or so.
Now, for diving, in warm water I'll be using the 2 pieces all the time, besides the thermo factor, it's a protection against sharp reefs and corals etc. There were times when my elbow was cut because I didn't put on my jacket, and I didn't realize it until I left water.Then for cold water diving, 1 piece 7mm.
Some people are using 3/2, arms and legs 2mm... it works fine for them for diving at warm water or whatever watersports they play. I've never used 5/3 nor 7/5, so I don't know. At my area 7mm for cold water, 5mm is not enough.
Finally, anything > 9mm is hard for you to move around. Time to get dry suit.
Oh, by the way, 2 pieces are easier to put on.
Hope this help!!!