I am from austria and I like vienna more in the sumer.

In Summer the city have the typical summer feeling. Everywhere nice flowers, proper parks, many tourists from all aover the world. You can walk trough the Kaerntner Strasse with icecream in the hand or sit in a open air gastgarten in Grinzing and listen to "schrammelmusic".
If you are lucky you can listen open air concerts in front of the rathaus, or you can visit europe biggest city festival (Donauinselfest)

Winter is also nice, (chritmasmarket) but If i have to choose - I would choose summer