I started diving in South Dakota in 1985, including liberal amounts of ice diving and extrensive diving in deep alpine lakes where the temps below 100' are pretty much always around 35-40 degrees. My first technical diving experiences were in the Great Lakes, including Lake Superior where the bottom temps below 100' are once again sub 40 degree, and I've spent the last 10 years doing wreck dives along the astern seaboard and cave diving in N FL and MX, with some scattered spots of pretty fish diving in Bermuda, south Florida and the gulf of Mexico.
Here's my take on it.
A dry suit is nice, and nothing beats a well made dry suit with heavy under garments in sub 40 degree water. Except maybe a dry suit with electrically heated undergarments.
However, on the other side of the coin, there are fewer things more unpleasant to put on during a rough boat ride in 6 ft seas on a hot summer day. Similarly, a dry suit isn't much fun on 95 degree day with 90% humidity. It's also largely self defeating as by the time you actually get in the water your undergarments are soaked in sweat and are less effective anyway. if they are not yet soaked, the humid air in them will soon condense on the inside of the suit when the shell is cooled by the water.
And, you pay for diving dry not just in higher acquisition costs but also with greater drag in the water. Trilams are the worst in that regard, as the material doesn't stretch so you get folds and wrinkles that add drag. For warmth you need enough gas to loft the insulating undergarment, so you've got more of a bubble to manage and/or you trade warmth for less gas in the suit during the dive.
I'm still a fan of a well made and properly fitting 7mm neoprene dry suit in extremely cold water. Given that the "shell" is insulating, you don't have the condensation issue, so with a light wicking and insulating under garment, you're toasty even in 35 degree water. They also swim very much like a wet suit. On the negative side, they are like giant swamp coolers between dives as the nylon outer fabric dries, so you need to either take them off, or cover them with a boat coat during the surface interval in cold weather.
In the middle, you have shell suits that are made from stretchy materials to allow the necessary freedom of movement without extra material, or you have a shell with a stretchy outer cover like the White's Fusion. They swim much like a wet suit and will accommodate a wide range of undergarments without increasing drag or restricting range of motion. But they are work to put on.
The ability for a suit like a Fusion to accommodate different undergarments offers the advantage of multi-season flexibility. Lots of dry suit divers will preach the virtues of their one and only dry suit, but the fact is that a trilam dry suit that fits well in warm water with lightweight underwear will be too tight when you put on heavy cold water underwear, and/or you'll be sacrificing full range of motion. The opposite is also true as a dry suit that fits well with cold water underwear, will be too lose and have excess wrinkles and drag with a light weight undergarment.
So a dry suit isn't the answer to everything, unless perhaps it is a Fusion, and those take patience to put on.
Instead, you need two dry suits, or a dry suit and a wet suit.
In terms of wet suits, if you've got the cold water covered with a dry suit, a 5mm one piece wet suit should cover everything else. They are easy to swim, they keep the wet suit compression and resulting swing weight on the dive to a minimum and work well in temps from 70 to 85 degrees.