May can be a beautiful time of year to visit. The weather is getting warmer, the flowers are starting to bloom and the farmers are getting onto the land. We can have some days that feel like July but also some that still feel like April. You need to come prepared for both! You can look at this chart for the average temperatures in May. Traffic tends to be lighter and many businesses operate on their off season or shoulder season rates.
I would have to say that September is my personal favourite. The weather stays beautiful throughout the month and there is always plenty to do. The days are still quite warm but the evenings are a bit cooler. Towards the latter part of the month, the leaves begin to change into amazing shades of red, orange and yellow. And the farmers are back on the land harvesting the crops that were sown in May. It also brings with it my favourite event all year! The Fall Flavours Festival is a unique, month-long culinary event featuring celebrity and local chefs. More information for September 2018 will be released in the spring.
I'm not sure I was able to help you with a decision! It's a very tough question to answer. I'm sure whichever month you choose, you will enjoy every moment you are here.