I'm not an expert but here is my experience so far after diving in Monterey.
Originally I used the rental wet suits - they are 7 mil, farmer john style. So chest area was a total 14 mil with legs and arms left at 7. Not too bad, but fit was questionalble and I did feel the water coming in, etc.
Did my first boat dive in the rental wet suits and to be honest, I had to abort the dive because I never got my breathing under control. Fear and cold I guess. I'm leaning more towards the cold since it was my first dive in the area. All previous dives off boats are in warmer waters and using 3 mil shorties...
In June I picked up a better fitting suit made by aqua-lung. 7 mil full suit with a 3 mill chest and hood combo that either can be used inside or outside. My next dive in Monterey off the cypress sea was better and I was actually able to make all 3 dives that day with comfort.
I started to notice the temps on the 3rd dive for sure. That day the temp was 47 degrees at 94 ft.
I'm on a budget but if I was to dive more often I would go with a dry suit. I just think dive times tend to be morning, it is cooler, water is cold anyway - might as well be dry and enjoy the dives more.
Dunno of this helps any - but I also wanted to add the part about how often you dive and in what water temps. Because it may make some sense to move that 400 bucks to a dry suit kitty. I know I'm wishing I did anyway.