I have the Pinnacle Seal (3mm Merino) which is a great 3mm WS for the 3mm range of water temps.
It will keep you very warm on repetative dives within the 3mm temp range. The same for the Pinnacle Fusion (5mm Merino).
Pinnacle has great products, both wet and drysuits (pricey).
I had a Pinnacle Cruiser 5mm and my Seal was warmer IMHO. That's why I switched to a Fusion for the 5mm range, but then I went out and bought a Pinnacle Drysuit EVO 2
, and now I don't dive wet that much anymore.
I would determine the temp range of the water you will be diving in the most, and go from there. The price difference between the Seal and the Fusion is very little, unless you are considering the Elastiprene for the stretchiness factor, then your talking $$$.....