I prefer to ride on a sans not too wet. To me, horses work better and easier on sand. For the temperature, between 10 and 20 degrees is the best. I have already ride when it was 35 et it is unpleasant for me and the horse.
I feel more comfortable on sand and most of the time horses also :)
Between about 10 and 20 degrees is the best temperature. After it is getting a little bit hot.
I ride a lot on sand (dressage, jumping) but also on grass (cross country) and grass can easily become dangerous in canter. It is hard to have a good terrain, without holes, that is not slippery... So I advise you to ride on sand when possible.
Sand! But sometimes the horse sinks into the sand, which is dangerous for tendons and articulations. If it is too hard, it is also not recommended to ride a lot on it.