Initially, I opted for the 7mm booties, but had to switch to the 5mm when I found the fit too tight when paired with my swim fins. I found 5mm to be sufficiently warm for a two hour set, but there was a notable difference with slightly less neoprene.
One major contributing factor was the split toe. I realize there may be some benefit when it comes to board feel, but in waters these cold, you won’t feel anything if you can’t keep your feet warm. Like the claw gloves, isolating a single digit exposed it more quickly to the cold and was the first thing to cut my session short. That said, it was worth it since I had a much better fit in my fins. Additionally, like the gloves, I sized down which further contributed to the degradation in performance. To remedy, I plan to replace with my specific shoe size and migrate to a round toe.