Disk or Rim Brakes on a Road Bike

Maintenance with discs is easier. Just don't lube the discs. They do cope with buckled rims better which may be a virtue and of course they won't wear the decent mileage in crappy weather and it might be. I was getting less than 3000 miles out of a front rim on mixed gravel and tarmac. Harder than most admit ably. Costs etc are much the same. All this depends on your riding style.
Ignore weight unless you weigh the same as Froome and race as hard. Other wise its vanity only.
1 nice virtue is that it is easier to swap between wheels, say your best pair and another as rims have different profiles which will be more noticeable than the tiny difference between discs.
I wouldn't worry about the bandwagon issue, many of us have been on it for several years. (about 6 in my case). Nothing will be redundant as QR disc front wheels have been around for decades and still will be. Plenty out there if you are worried about a QR fork becoming redundant.
Many people may not be worried about discs and find that their braking is fine without them. That's the only negative to discs for no weight weenies. why change what you don't worry about?
No one can tell you what you need, sorry.
However as I doubt you would buy a bike without a ride, why not try both? As the bikes you suggest are so different you obviously have an open mind so are not just tied to those two.

Having ridden both extensively, I honestly think I'd never commute in London on my road bike.

I did 35 miles a day on my mountain bike (with slick tyres, but it's a hard-tail) riding from zone 5 south London to Holborn and back for the best part of a year. The state of the roads in London, along with the godawful drivers + traffic in many places on the route - I'd never put my road bike through that. On at least two occasions the excellent braking of disks saved me from going over the bonnet of someone who pulled out in front of me.

Around my way at least, I'll stick to larger tyres and enjoy the extra safety/control of the hydraulic brakes. Even with upgraded aftermarket pads on my road bike, the Shimano Tiagra brakes on my road bike are nothing like as good as my MTB, and that's not even taking into account the extra grip you have from the larger tyre contact area and lower pressure.

Maintenance of disc brakes (on an MTB at least) is actually pretty easy once you have taken one apart, there's loads of stuff online about it. Just don't do what I did and loose the little rubber washer between the calipers housing without noticing... Dot4 fluid all over everything.

Cheers,

Well, a couple of comments: you'll flat less with "commuting oriented" 32s. I use the panaracer ribmo's, and i'm not sure i've ever had a flat with those. i can also ride gravel with them- they're nice tires, plenty fast, durable etc. there are likely plenty of similar ones out there. i'm not sure why you think it would be any harder to change a flat on a disc brake bike- i guess it might be if you had thru axles, but with QR's it's exactly the same. I guess you have to make sure you put the wheel back on with the rotor on the right side ( : - ) ), but I suspect you're up to that challenge. disc pads are pretty comparable to higher end caliper pads, pricewise, at least here in the colonies.

i've got a carbon rb with calipers, an alloy cross bike with cantis, and a ti gravel bike with discs, which i tend to use a lot also for just doing hills before work. the braking in the wet and in the snow is definitely nice with the discs.

One thing to consider about disc brakes is that you will be able to fit a wider tire. Very few caliper brake road bikes will accept 32's, and many will not fit 28's. You will be amazed how much more comfortable you are commuting on crappy roads with some nice 32 slicks. It's not at all hard to find disc brake wheels these days- check out the HED Belgium disc wheels, for example.

I have shimano hydraulic disc brakes on my Cube hybrid which has an Alfine hub gear and so is fairly heavy, and have been grateful for the extra stopping power, especially when I ride with a cycling group which includes some members who can be unpredictable; I think the same would apply in town with errant pedestrians etc. Taking wheel out for punctures not caused any problems. My road bike has Tektro callipers, which are OK, and better with replacement cartridges, and if I was getting a new road bike I would be happy with Shimano callipers rather than adding weight with discs, which would be mechanical not hydraulic at the lower end. Hope this is helpful.

The ease of maintenance of disc brakes is much more variable than rim brakes, primarily because almost all rim brakes follow the same basic design. There's some variation in disc brakes (hydro vs cable operated, different adjuster designs on some models etc). That said, none of them are really that much more difficult than a rim brake.

I still think it's maybe a little early to jump on the disc bandwagon just yet. Outside of handbuilts, there isn't a lot of choice for wheels. It's also not clear if the industry will move to standardise on thru-axles like the mountain bike market seems to be doing.

The two bikes you've picked out are very different beasts, and I wouldn't normally expect those two models to end up on the same shortlist. The Synapse has more upright geometry and is designed for comfort over long distances. The Canyon is more argressive. Cannondale's CAAD10 is probably the more appropriate comparison in their range. I would think more about how you intend to use the bike and get one that fits best, rather than worry so much about the brakes.