The S5pro is great for landscape work particularly because it can simulate Fujifilm's transparency film. As for the softness that's so spoken about, it is not that bad (in fact not at all) and if you shoot RAW (which is what you really want particularly if you do landscapes) you will get some great results and no softness. And no, the megapixel difference between 6 and 8MP in print is negligible and even between 6 and 10MP it is only 2 inches. There was an AP issue that dealt with this some time ago.
I have many prints from my S3pro (the previous model) which has the same sensor and they print very well at A3. But with the S3 and S5 the sensor is perhaps nearer to 8MP in terms of resolution than 6. At times you can also interpolate the S3 and S5 images with satisfactory results to 12MP. Even better results by using a very good lens and low ISO but I have done that at ISO400 and the results were very usable at 12MP.
I hope this helps!
PS: I would say the sharpness is pretty much equal all things being the same but the D80 is 10MP which will give you a bigger print or if printing at the same size more detail. But if you don't need the speed, I think the S5 is your camera as you like people photography.