I would guess the 5mm will be fine. I have never dived the Red Sea in November, but have dived there at the end of October several times and everyone is wearing wetsuits. At the end of October suits vary from 3mm shorty to 5mm full suit. It depends a bit on the route and number of dives as to how cold it feels. Having a looked in my log book lowest temperature I have recorded at the end of October is 23 degrees C, this was on one of the wrecks in the north, last year in the south it was 25-27 degrees. For the northern routes the water is always a degree or two lower than the south. I was on a liveaboard a couple of years ago at the end of October with a lady who was wearing a drysuit, she seemed fine and wasn't too hot. During the day the wind tends to still feel warm, but it can feel cold when coming up from a night dive.