Having ridden both the dual and mono 65's, I opted for the Mono 65, which essentially is the mono 80 with a smaller engine (though you only really feel the difference in top gear pull, the 80 doesn't put out all that much more HP).
I rode an LS a while back I was considering buying, and was blown away by its quicker handling and lively engine compared to mine (which had given away 2hp for unleaded heads) which amounted to a nice package, but the choppier ride had me hankering for a longer wheel base, which is why I eventually shelled out for a 90/6. I decided I wanted a bike for longer distances, and both my mono and 90 fit that bill. I know there are plenty who own dual 65's who ride far, but I just like it with a bit more comfort!!
Advantages are a longer wheelbase, unleaded heads, later machine, overall sweeter engine, parts that fit nearly every other BMW model. Disadvantages are not as round town agile, not as distinctive as a bmw model.
My next Beemer might be an LS if I can get one, but why don't you get both??
Bill.....................;-)