Buying a road for the first time!

Cable discs are fine and reliable regardless of the weather. Not up there with hydraulics but fine enough.

The latest Claris groupset is also fine, I have this on one of my bikes and the shifters are now the same shape as Ultegra, 105, Tiagra etc.

As per previous posts, comfort will be as much about getting the right size and being able to fit larger tyres at lower pressure - rather than any great difference between frames. That said, the Cannondale CAAD Optimo will probably be the nicest/lightest frame of the ones you suggest.

Unless you know what you're looking at I wouldn't go second hand. Or it is genuinely a barely used second hand bike. Even higher quality groupsets don't feel as sharp or as smooth after a bit of use.

Cable discs are fine and reliable regardless of the weather. Not up there with hydraulics but fine enough.

The latest Claris groupset is also fine, I have this on one of my bikes and the shifters are now the same shape as Ultegra, 105, Tiagra etc.

As per previous posts, comfort will be as much about getting the right size and being able to fit larger tyres at lower pressure - rather than any great difference between frames. That said, the Cannondale CAAD Optimo will probably be the nicest/lightest frame of the ones you suggest.

Unless you know what you're looking at I wouldn't go second hand. Or it is genuinely a barely used second hand bike. Even higher quality groupsets don't feel as sharp or as smooth after a bit of use.

http://www.betterbybicycle.com/2016/11/are-internal-gear-hub-bikes-secre...

I agree to this statement.
I often use a Avanti Blade 8.
Truck suburb, industrial roads. 38mm, 35mm.
This morning left my phone at home. So dropped tool kit, panniers and heavy clothes at work and legged it. Cruised a couple of kms at 40kmph then realized my keys were at work.. Point being I didn't realize they went that quick.
Speedo is out. 7.2 real = 7.5 read.
Then looked into dropbar mod today, realized I'd lose mud flap and brake purchase.. Nice with bull horn ends anyway because sits upside down clear of lights.
I don't maintain this bike, it maintains itself..
Just a thought because I read education. BTW I was in 7th of an 8 speed. It also easily climbs 10% loaded.
I think I'm building another bike soon.

for same price on fleabay, if its anywhere near your size, free postage and it has mudguard eyelets as a bonus https://www.ebay.co.uk/itm/2017-Genesis-Delta-20-Size-54cm-Medium-16/312...

As above the Tiagra 4700 series is very good, it's akin to 105 5700 series IME, not quite on a par with Ultegra 6700 nor 105 5800-11 speed either though the design is more like the 11 speed.

Of the three you've listed the Merlin is better value, you can also get another 1% cashback using topcashback.

Tiagra groupset is really close to 105 and costs £300 alone so you're getting a fair bit of extra bike for your extra £200. Claris groupset is awful in comparison.

One other to throw at you is the Planet X London Road Apex 1 - you only get 1 cog on the front but based on your previous 1 gear you'll be doing well!

Without apologies, I think I BS too much..
A road bike that accommodates 35+ mm tyres, mud flaps and a pannier rack would be my choice of an ideal commuter.
28mm is pretty much perfect. Fast, carries some load yet here is the middle of winter and my punctures were almost always during very cold wet nights.
I don't think I punctured during summers. But if you find a bike that fits wider tyres you can still use skinnier tyres, not visa versa.
Discs look awesome, don't have to get past tyre if ever punctured.

For me it would be the Merlin or Decathlon.If you feel the need for disc brakes and are happier with a lower tier groupset then its the Triban.And the frame can accommodate wider tyres.If the bike was purely for weekend recreational use then I'd choose the Merlin,but commuting,the Decathlon makes more sense.

Sensa has the best kit, but no idea on whether the frame is any good

Decathlon has discs which may be useful for commuting, but if you've been riding single-speed this might not be a factor. Its also the only one that will take racks and mudguards

Cannondale is the least practical but probably the most grin-inducing. Will unfortunately attract more attention from thieves than the other two.

I like wide tyres.
I started regularly commuting about a year or 2 ago..
I like my bmx. Light weight.. Doesn't puncture. Opens up paths. I guess about 15kmph average.
I upgraded to an Avanti Blade flatbar road bike. Then upgraded that to drop bars. Replacing the 28mm tyres with a 38/32mm combination (the widest that would fit) improved my want to commute. On the smooth flat it might be slightly slower yet climbing, descents and bumpy roads aren't. It's comfortable, it carries work tools using panniers and I'm yet to puncture.
I wanted a fixie and found a second hand Avanti Blade with an internal rear 8 speed hub. I couldn't easily change the wheel to a single speed so I fitted the widest tyres that would fit (38/35mm combination) and that's my most used bicycle. That bike scored mud flaps, gets ridden all weather, all times of day/night, carries loads via panniers, ratios climb 10% grades and comes down fast too.
I found a fixie, widest tyres I could fit are 28mm. That's my favourite bike. Aero, fixed, 7kg. Yet because I dislike punctures, that hangs around until summer.

I ain't the fittest.. Average speeds tend to be about 15/20/25/30 kmph regarding bmx/flatbar hub gear Blade/ dropbar 2*9 speed Blade/track bike with brakes.

I love thinking I should afford a titanium road bike with discs that fits 700c*38mm front/rear.. Laughing .. The 32mm was so flat last ride as it'd been hanging most of a year yet I was late so I jumped on and rode. After work I pumped it up using the hand pump.. 28s are more difficult to do such..

I'd look at 35mm+ front rear tyre width clearances.. In my part of the world, 32mm+ makes commuting faster on most days yet... Best bit.. More reliable, more fun, easy to speed with, basically benefital to commuting needs.

I like the second bike.. Because it's cheaper and more so.. Because it has disc brakes and I've never had a bicycle with disc brakes. I guess they're awesome.. Wear a disc not a rim.

If my dropbar Blade breaks I'd replace frame with a cost effective cyclocross disc brake unit and transfer components that fit.

Everyone is different. My mate prefers 28mm due to longer distances. I prefer wider due to consistent wear. He wears rims, punctures and a lot more kms than me..

My 2 cents. Best luck