Choices, choices... hard to choose the right Canyon Ultimate

Both C and D seem to make for pretty decent all around bikes and pretty good climbing bikes, with C being a good comfort option with the Di2 and the easily adjustable geometry and D potentially quicker in the flats with the aerocockpit and the shallow-depth carbon wheels (as long as the aerocockpit geometry is pretty close to what you need). 

As no one seems to have mentioned this, I think the timetrialling bit may be relevant. If it's something you might be actually interested in I'd definitely go with C, which is pretty much just as good a climbing bike as D and would probably make a decent TT rig with an easily removable cheap clip on bars / shorter stem combo (instead of an expensive and impracticable new cockpit and intractable cabling nightmare on D), and a eventually a pretty good one with a some Di2 TT switches and maybe even some proper deep section wheels that you can also use in flatter outings.

C.

Difference in weight SL to SLX is negligible for, what, £800? No brainer. Especially since the SL comes in a lovely blue.

I would definitely go discs if I were you, and (even though I have electronic gears) I'd go regular gears and get better wheels.

Agree with unconstituted, aerocockpit is a bit of a faff, and the garmin mount canyon sell is awful. I'm probably going to flog mine and go back to a regular format with a bit more adjustment.

Part_robot suggests that buying a frameset might be cheaper, but I doubt it, Canyon's builds are very good value compared to their frameset prices.

Keep an eye on the outlet store.

Aerocockpit + Di2 is a pain in the arse. You have to unscrew a panel at the front to recharge the battery or index the gears. It gets worse once you stack a Canyon aero mount mount onto the same screws. Fiddly nightmare, with cables everywhere. I stripped the missus' frame and put eTap on instead. Also, Di2 is fiddly with gloves on, the shifters aren't well thought out. WIth mitts though Di2 is lovely, but can't compete with eTap for properly gunning it and knowing that when tap your right hand you're going to get the gear shift you want. As for mechanical v electronic. There's no comparison. I've ran Dura Ace 9000 and it was beautiful, but electronic is a different world. It's a luxury item. If you have a bit of cash and want to make your ride a bit more special, then go ahead, treat yourself. Or don't and do it in your next life instead. 

If you're smart you can buy some Canyon models, sell off the wheels and groupset and stick your preferred parts on them and the total cost of the build is actually surprisingly good. Well, it was like that before September when they hiked the prices up a solid 400 quid on average cos of Brexit/pound devaluing. Would need another proper look at them to see what's what, but you can do that yourself. Check the eBay prices for selling the wheels on the mid to high end builds. Same with Di2 - though remember, if you want to remove the internal Di2 battery from a Canyon, you need to remove the bottom bracket as well and reinstall it. So you need to be a fairly clued-in home mechanic. Also check to see if you can fudge the checkout so you pay in Euros. Might take the cost down a bit. I'd have a crack at it myself now to see but have to go shortly.

Also, the aerocockpits are a love or hate affair. Canyon like to ship them in width and lengths related to the frame size, and if it's bad for you, then you have to ship it back to them and exchange for the size you want (for free though). Canyon will start selling them separately this year however if you decide to do your own build. They are lovely that said, look amazing and are actually really nice to climb on the tops on as the tops aren't super flattened like say a S-Series aerobar.  

Or buy a frameset and build up what you want. Cheaper and you'll hopefully never have to deal with their returns department. That's what I'd do anyway.

Di2 or no, get disc brakes.

You don't need DI2. The best mechanical groupsets work just as well.