The arrival of snow is rather unpredictable: snow statistics of Finnish Meteorological Institute (these stats are sadly only available in Finnish) tell that since 1961 on average the first proper snow cover in Helsinki has came on November 19th, but then the earliest date recorded is 11.10.1973 and the latest is 12.1.2014, so there is quite some variation and you never know in advance how it is this year. And since the sea still pretty much certainly open that time of year, that moderates the climate in Helsinki a little, so in late autumn the temperatures are not super cold, maybe little below freezing at times. And especially in October the sun may still warm up while it is up, in November less since the daylight hours are becoming fewer each day. But I think that how pleasant it is walk around depends more on how rainy (and windy) it is rather than which autumn month it is. And obviously you can predict these weather phenomena only few days earlier.
Anyhow, since you mention photography, I think October might be a better bet since there is more daylight and you most likely will see more colourful leaves on trees then.
I'm more into nature photography myself, but few places that pop into my mind immediatly are Suomenlinna (worth a visit in general) and Ateljee Bar on the top floor of Hotel Torni. And for slightly less urban cityscapes you might want to venture further away from the city centre and head to landfill hills either in Vuosaari (in the eastern parts of the city) or Malminkartano (in the north-west).