I had the 110 and loved it. It is a great lens but like you said it is pretty close to what you have all ready.
I had the 180, 110 and 50 ULD in my kit. Doing what you are doing I would have preferred using my 110 if I could so I recommend the 90 to you. Of course this recommendation is only if you won't be in the middle of traffic using it!
Of course if you start doing interiors then the 50 would be nice to have also. There is nothing wrong with the 65 but shotgun houses usually have small rooms.