Last year we went in the 3rd week of January and it was perfect. The crowds weren't too bad, and either was the weather, it fluctuated in that some days felt like early June, some felt like late April in terms of Toronto weather. The crowds were perfect, no line ups.
This year we went first week in January and we didn't find it too bad, there were a couple when it went to about 50 degrees f, but my husband and I laughed at the employees wearing coats and gloves. The crowds at that time were also very good except for a few days during the marathon.
I think I will only go in January now, as my friend took her son last year at March break and said it was terrible with the crowds.
In my opinion if you are looking to go at a time when you can avoid crowds go in January, if you are looking for weather go another time.
We had no rain both times we went, it was like a typical Ontario spring.