I've never owned a boy in Lambskin so I don't have any input there, but just wanted to say it's absolutely gorgeous!! My heart says get the Lambskin beige, but practicality wise (especially if it's your first one), get the black caviar. My first boy is also black caviar and it still looks brand new.
I got the black caviar lastly.
Actually I got a medium navy blue classic flap bag, and always want to get a boy too. Suppose I should get a lighter colour this time for boy.....probably i prefer calfskin than lambskin. Still I love the dark beige, hope I still can get such colour in future!