Bicycling

You're looking to exercise. That means expending energy. The carbon bike's reason for existence is to reduce energy expenditure. It's lighter and more aerodynamic. A side effect is that it's also a bit easier to carry although 2-3kg difference isn't a lot for that purpose. Unless you're looking for setting times against others, there's not much point to it for you. Then comes the ease of living with it. The alu frame will likely last till you're in your 60s without any special care from the elements. It'll likely need no treatment after crashes either. Paint scraped? No problem. The carbon on the other hand is much more difficult to live with. You can't ignore dings and chips in the paint. You have to inspect it for safety (ultrasound, etc) after significant crashes. Alu is much more usable in comparison and I'd take it any day over carbon, even if I had a high budget.

On a side note, don't be afraid to change the bike from its typical "road" shape. Different handlebar styles (go flat if you prefer it) and heights, tires, saddles, suspension seat posts (oh yeah 🫠), tweak it however it feels comfortable. Attach a rack for storage if you have to. Going 70k on a bike that is comfortable feels much nicer than arriving bent into a shrimp.

No idea about the bikes (I have only ever owned one). I would suggest that buying from decathlon means you have a shop to go to for warranty repairs if something goes wrong.

... I also have a distrust of carbon bikes having seen a front fork snap on someone before.

I don't have any experience with those brands and I'm still pretty new to road cycling, so I can only give my general thoughts.

You're right that you don't need a carbon frame, but I recently switched from aluminum to carbon and the difference was eye opening. Just the amount of vibration that it dampens on pavement has made a massive, immediate, change in how biking feels to me.

One thing I would consider before making a purchase is the possibility of upgrades/changes that you might want to make in the future. I haven't looked at the bikes you linked but, as an example, if you were to buy a bike that couldn't accept a standard round seatpost and stem, then you wouldn't be able to buy replacements like the redshift shockstop system, which are fantastic.

But other than that I wish you good luck =⁠-)

Edit: oh, also, this: https://youtu.be/sO6TONX4BN8

Got to say, I'd be tempted by the carbon of the two. It's going to be an open mould Chinese frame, but there are lots of good ones of those around. The big consideration at this point is getting a good frame and wheels - you'll slowly upgrade other components over time anyway. Both bikes are 9 speed (one is definitely Sora and the other sounds like Sora too). The Triban is heavier and, although the wheel weight isn't given for the other bike, I suspect they'll be a bit lighter.

Ultimately, the right bike is the one that would put the biggest smile on your face, and without testing both it'll be hard to know which that is. But do have a good look at the carbon bike, and consider upgrading the brakes to Spyre hydro calipers before long

I have a Triban 120, Id say go for them, they're very good entry level bikes.

You could get a Triban 520 under 1000 euros as well, you get a Shimano 105 groupset which is kinda the gold standard for value.

I wouldn't go for a carbon bike that cheap, those thing tend to crack if they're not properly made. Maybe check out something like a used Orbea Orca