Hi Timothy! From my experience and from what I learned, Europe has lots of facades. That's the beauty of the continent. I've only been in Vienna and Milan and both looked great with the snow in winter (only met them in february), but the common sense indicates that the gardens and the public places of Vienna can be really seen in summer or spring. Milan could be the other way around. Their parks are great but there's no real difference between those parks and other parks you might find in other cities inside and outside Europe (in my humble opinion). While all the guides for tourism say that summer and spring are the best time to enjoy Europe, I would say that it depends on what you're looking for. I remember walking in Venice for the first time in my life with a snow-storm at night, with almost no people in the streets, and it is one of the most valuable experiences I've ever had. The moment was magical. Same for the Amsterdam.
Whatever your choice is, you're gonna have a great time, that's for sure.