I posted in the other thread but deleted it and re posted it here. Though I will say, mstateglfr's advice is excellent.
Which ever bike you choose you will be happy as they are both great bikes. It is a win/win in this case. I won't say one rides better than the other, they just ride differently. The carbon bike does have a slight edge in features such as the ISO speed and the storage box but it doesn't mean you will enjoy the aluminum bike any less. On a gravel bike I am not convinced that frame material is the be all/end all in ride compliance, but it is the large volume tires that bring the comfort. With that said I am a carbon fan. I love the lightweight aspect of it, the sound of the road/trail under me, knowing I have the latest technology, knowing I never will want to upgrade the frame and small part comfort when using less volume tires.
Having two sets of wheels is a good idea, I do this myself. You need to factor in the cost of new brake rotors, cassette, tires and tubes. It is best to have the same type of hubs so that you don't need to re-adjust derailleurs or brakes but I have different brands and don't have this problem with different wheels.
For me, it really comes down to money. How much are you willing to spend? Can you afford to buy the carbon and if you can afford it should that money be spent better somewhere else? The price difference between both bikes plus the second set of wheels can get up there.
The last point is about regret. Are you the type of person who has regret in purchases. Do you ever say "I should have bought. . ." Use your car as an example, do you say I should have purchased those extra features so I regret what I bought. This has come up in my own decision making process. I was looking to purchase a new gravel bike with mechanical because it is what I can afford but decided that I would regret it because I love Di2. Right now Di2 is in short supply like my disposable income right now. So, I will wait and save, defer purchasing a new bike so I don't have regret.
This is only my opinion and it would be good to hear others and their thoughts on this subject. Good luck with your decision.