One of the guys in my LBS rides the Domane ALR2 as his daily commuter, he says it's a great bike, and having the Domane ALR I'd agree. The rim brake version is limited in terms of tyre size and mudguards compared to the disc brakes but that will be the case across all rim braked bikes.
Mudgards are very very good in UK/Irish/Northern European climates though so worth keeping in mind. The CAAD optimo will allow you to fit them, the Orbea won't. The Domane is the only one which would allow you to fit a rear rack.
I can't comment on the Orbea or the Cannondale as I'm not particularly familiar with either, but a few points about the trek - the wheels are the stock wheels for their alloy bikes, and they're decent, not particularly heavy and are quite durable - they definitely wouldn't need upgrading, the tyres are "OK" for the price, but nothing spectacular but they roll well even if they do lack decent puncture protection and are a little heavy. For your first road bike you won't notice them, and that's probably the best thing to say about them. If you put mudguards on then you'll probably need to downsize the tyres anyway. A point to note is that my criticisms of the Bontrager tyre are exactly the same as those I would have about the Vittoria Zaffiro tyres on the Cannondale or Orbea - the difference is the 28mm on the Trek will be slightly more comfortable than the 28mm, and their grip is better in my experience - the basic Zaffiro is a bit underwhelming.
Overall I think the Trek is the better bike for a number of reasons, firstly to me, the blue with the silver logo looks spectacular - definitely looks like a much more expensive bike. Secondly the lower price means you have a bit of money left over to upgrade the things you'll actually notice such as the tyres, get lights if you want them or mudguards or other accessories, you could even, and this is contraversial, do something non-bike related with the money. Lastly I think the Trek will take upgrades a little better in the long term - I upgraded my Domane to full Shimano 105 11 speed - which is their best value/performance groupset and the frames are very good and take an upgrade really well, so worth keeping in mind for the future.
To be honest - whilst I've written a fair amount so far - all three of those bikes are good, and I doubt you will regret buying any of them, go for the one which you think looks best, in terms of performance, they are all good.
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