Based on my experience with winter/spring whitewater rafting:
– Drysuit plus fleece/polypro can be much warmer and more comfortable than a wetsuit. But you need some time to get used to being constricted by the seals at neck, wrists, and possibly ankles, depending on design.
– Don't trim too much off the drysuit seals hoping for more comfort. Trim a little at a time, wear it for a while (like an hour), then think hard about the next trim. Trim a little too far, and you get to replace the seals. Not cheap or easy.
– Drysuits are relatively fragile. Put a hole in a drysuit body or seal, and water & air gets in & out, and your warmth drops rapidly. For Tough Mudder, consider buying the toughest drysuit you can find, not the lightest.
– Most drysuits are terrible for regulating temperature when you heat up from sun or exercise. If you open the zipper, you risk getting wet (and cold), and jamming the zipper with mud.
– Some people carry a short length of 3/4 to 1 inch diameter plastic tubing. Wrap around your neck in a circle loosely, then roll the neck seal over the tubing, to allow heat and sweat to escape for a while. Helps to get the drysuit on and off, too.
– You will sweat inside your drysuit, and the sweat will pool near your feet. Goretex and similar miracle fabrics help, some, at considerable extra cost.
– Drysuits with built-in feet can be a little warmer, but the sweat pools at your feet, which can cause problems. Drysuits with seals at your ankles result in water pooling around your ankles, which is relatively easy to drain by tugging on the seal.
– 24 hours? You definitely want a drysuit with a "relief zipper" designed for your current gender. Getting in and out of a drysuit is hard work, time consuming, and risks tearing the seals.
– Get a drysuit with covered seals, typically cuffs with velcro. Seals are the weakest link.
– A dry top plus dry pants might be a good option versus a one-piece drysuit. Easier to regulate your temperature, easier for waste management, but won't keep you as dry, because sealing around your waist is difficult.
– A wetsuit will be much tougher than a fabric drysuit, but not as warm.
– A reasonably thick wetsuit will make it harder to run and climb. Practice.
– Watch out for wetsuit chafing and rashes behind knees, at groin, inside elbows, and armpits. Consider lubricating those areas before starting, and having more lube available during the race. Don't use petroleum-based lubes! (Where have we heard that line before?)
– In theory, you warm up the thin layer of water inside the wetsuit, and all is well. In practice, stay as dry as you can as long as you can. Cold water is still cold water, and it doesn't stay put when you are active.
– Wetsuits with a fabric outside surface are much tougher than "raw" smooth neoprene. However, the fabric retains water, which evaporates, which quickly chills you. That's why windsurfer wetsuits are raw-smooth on the outside. Inside, you need the fabric to get the suit on and off in under an hour.
– I've used a "splash jacket" or "paddle jacket" over a wetsuit to help keep water out, and act as a windbreaker. You can add and adjust warmth with fleece or polypro between the wetsuit and the splash jacket.
– Consider a custom wetsuit. A wetsuit that's too large in major areas (arms, legs, torso) is almost useless, because the cold water flows right through. Too small, and you constrict breathing and blood flow, generally Bad Ideas while exercising hard in cold weather. If you can find an off-the-rack wetsuit that fits really well, go for it, but don't settle for OK.
– Wear only the thinnest polypro under a wetsuit, or nothing but minimal underwear. Fleece or thicker polypro under a wetsuit holds more cold water next to your skin, and it can't evaporate.
– You need to keep your hands, feet, head, and face warm, too. Don't assume a warm body will keep the rest going. Those temperatures are in the right range for frostbite, chillblain, and trench foot. Under those conditions, you want neoprene gloves or mittens, socks, and a full hood that covers your neck.
– Running shoes or water shoes over neoprene socks work well, you'll need larger shoes than normal. I've never found an insulated shoe that worked well – usually bad fit, traction, comfort, or all of the above.
– Choose shoes or sandals that hold as little water as possible. More water is more weight and more cold.
– Difficult trade-off between "sealed" shoes or shoes + gaiters to keep dirt and stones out, or more open designs that make it easy to dump the dirt and keep moving. I've settled on Chaco sandals plus neoprene socks, with lots of practice balancing on one foot while dumping stones from the sandals. YMMV.
– Ice rescue suits, survival suits, and immersion suits are really expensive, tough, and warm. I have never seen one, no opinion.
Hope this helps.