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Alfine bike will be heavier,
This is probably true, but I suspect the weight penalty is actually pretty minimal and it's just the concentration of the weight in the rear hub that makes it seem significantly heavier.
I don't tend to use my Nexus (basically the same as an alfine 8) in 4th gear as despite being the 'direct drive' gear, I feel thatin this gear there isa bit of drag. I don't notice it in other gears. I've had an Alfine 8 in the past and it felt the same, I suspect there may be a gear in the 11 speed hub that feels similar.
My Day One wears full guards and gets a lot of wet weather use (commuting). It also has a dynamo lighting setup which I feel is probably it's weak spot in the wet. I'm glad it's weak spot only affects the lights and would be cheap to fix. If you see what I mean?
Why do you want to replace two bikes with one?
Alfine bike will be heavier, gear changes are evenly spaced, and will feel odd for a while, as you'll never find that gear that is just right like you can on a normally geared bike, and there will be about half the gears that you never really use.
Replace the SS with the di2 if you want an everyday commuter and keep the cdf for your sportives etc....
Also why di2 and not just common or garden wired alfine? Unless you can get a good deal the jump in price is kinda steep...