I would go for the 40 if i were you...
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Please buy it! =)
I use it all the time and i can't live with out it.
Using the 24-105 at the wide end using manual focusing for landscapes/architecture is also a good idea. Drop the ISO down to 100 to maximize DR/sharpness and stop down the aperture to f/11 or f1/16 to maximize DOF. If you are ok using this workflow for landscapes and architecture, then the TS-E will be a great addition for you.