Computing the average hours of sunshine at given location&orientation I made it my geeky aim to see which of two flats that we are considering moving into is sunnier. The first flat faces S and the second faces NNW; both are at the same height above ground and have similar terrain (buildings/trees) around.

Conventional wisdom would suggest the S-facing one is (much) sunnier, but I am not sure if this is really so, since I don't think it is the case that the Sun, when it's above the horizon, spends most of its time in the Southern-hemispace/hemifield. 

Furthermore, I'd like to get a more quantitative estimate of how much direct sunlight each flat gets per day (averaged across seasons). How can I use tools available on the Internet (probably GoogleMaps-based) to find this out?

My only solution to date was to generate Sun trajectories (elliptics) for each flat, using the SunCalc.net website; these suggest the NNW-facing flat hardly ever sees the sun, except around the summer solstice. Is this a reliable estimate, or can anyone suggest another way to solve this riddle?
Between September and March (ie winter, assuming a Northern Hemisphere location) The sun rises in the South East, is Due south at midday and sets in the South west. Your South facing window would get sun for as long as the sun is up. The North West facing window would only get a glimpse of the sun around sunset (and it would then be low and weak)

Between March and September (Northern hemisphere summer) The sun rises in the North East, it is in the south between roughly 6am and 6pm, then sets in the North West. Your Southern window would get 12 hours of sun each day. The North West window would get some sun in the evening, and this sun would be low (weak but shining into the room)

Overall the South facing window would get a lot more sun than the North West window. The reason is that the sun is always above a point on the Earth that is between 23.5 degrees North or South of the equator. For anyone in the Northern Hemisphere, the sun is usually South of them. 

A quantatative estimate of this could be done, but the actual values you got would depend on the Latitude and precise aspect of the two windows. However the result from SunCalc looks to be robust. The North West facing window will only get a little sun on summer evenings, and little or nothing during the winter.