My experience at Forte Belvedere has been at all times of the day, and all times of the year. Forte Belvedere and Boboli was the place where we would hide when we cut school in my youth growing up in Florence. Since I used to cut school a lot in spring, I have extensive experience of the place.
The BEST views of Florence are actually from Forte Belvedere, not Piazzale Michelangelo. That is because Forte Belvedere is closer to the city center therefore the monuments and the skyline is so close to you that you can almost touch them. And yes, it is also less crowded (don't go in the spring mornings when Florentine high school students cut school though).
The only advantage of Piazzale Michelangelo is that from there you can see the Ponte Vecchio (if you are interested in photographing the bridge). From Forte Belvedere the bridge is not visible, since it's below you hidden by buildings. Other than that, Forte Belvedere wins hands down.