Two weeks in March or three weeks in July/August?

So we are planning a trip to Japan, and would like to go for three weeks because it is a big trip for us (we live in eu), but can only go for three weeks in July or August, but after reading a bit, I saw that it is really really hot during those times and that you can do less in a day because of the heat. So we could also go for two weeks in March, will this be better and will we be able to see more? Any other advice is also welcome!