Exposure protection from the cold is very much based on what you are personally comfortable with. Some temperatures feel cool to some divers and freezing to others. You will need to adjust your exposure protection to suit the temperature, the equipment type, and the type of diving you are doing. It is best to talk to a dive center and ask local advice for the area you plan to make your dives.
Generally, open circuit diving requires more exposure protection because you are breathing cold air. However, diving in water that is too hot can also be uncomfortable. Remember that if you are uncomfortable in the water, abort the dive. There is always another dive day!