Hi and welcome! What an exciting time for you!
I'd agree with the common view that 'girls love you but the boys are in love with you' -- I've always found the boys super-affectionate (in most breeds actually!) but of course this varies per dog. I don't really think there's much difference except that females require more careful guarding until of age to be spayed, ideally after a second heat. The thinking has changed on when to neuter and evidence suggests not doing it at 9 months or especially not at 6. Two littermates can be fun but TBH are not ideal. I've had littermates before and what tends to happen is even while really loving, they noticeably (and by contrast with the many many singletons I've had) bond very closely together and to a lesser degree with their humans. One can also be very dominating of the other, something not noticeable sometimes until in old age, when one passes away. Also, they require considerably more work because you will have to work to give them time alone and training and walks separately as well as together, as they can otherwise grow so dependent on each other that they get separation anxiety and also, don't get a chance to learn as well, or just well. They need separate housetraining too, which is a huge amount of work for the first few months with puppies, and if there are no older resident dogs to model this, it's even more work.
Now that said it can be great fun to have two pups of course! In the past decade+, we had two Pyrenean Mountain Dog (Great Pyrenees) siblings (not by intent, it was the particular circumstances) and we worked hard to do things separately with them as well as together. They adored each other and having the two (alongside the cavaliers and at the time, a Newfie and a GSD) all seemed no problem at all. BUT it's really now in retrospect, and now that we have had a new singleton Pyrenean pup over the past year, that I can see how much closer the relationship is between us and Finn, the new boy, and how much more focused he has been for training and so on. I'd personally never choose siblings again though I wouldn't say I regret having done so at the time. But I've longer perspective now and would agree with the view of most professional trainers and behaviourists that it's better to have a singleton and to add another dog when the resident dog is at least a year, ideally at least 18 months old, and fully trained and has benefitted from all that time of your focused attention to develop and learn. Hope that helps! There's no wrong decision here though. Oh the one other thing I'd add is: I'd not get two siblings again, as if one ends up with a health issue it is far more likely the sibling will too, and managing two together is tough. We lost our sibling Pyreneans to bone cancer, one quite young, and the specialist said she immediately suspected it in the second dog when she developed symptoms, because her brother had had it. Cavaliers sadly have a LOT of potential genetic illnesses and ideally I'd always go for unrelated dogs of different lineage, from fully health testing breeders (hearts, MRI, eyes, patellas, DNA tested for the things that can be tested for etc but especially MRI and heart tested parents).