There is some really helpful discussion here! I have to admit I really wanted Albus to join Slytherin since Snape is one of my favorite characters, and I think there are some poetic elements to having him join the House of his father's rivals. I never considered the sibling rivalry, or that his personality might place him in one of the other two houses -the wizard families appear to be sorted into the same houses for the most part - Sirius, of course, being an exception. While Albus certainly could have chosen Gryffendor as his father did, a part of me hopes Harry's words would reassure young Albus, and that he would remember them when his time came for Sorting.