Just thought I might provide the "child's" point of view (although I realise this will be different for each child). My brother is 2.5 years younger than me, and I think it's ideal. I can vaguely remember helping Mum out when we were younger, and we were close enough in age to be good friends. At school we were 2 years apart, which was good as we had some same friends, but also plenty of separate ones. I don't know if it's due to the age, but I can only a remember 2 or 3 fights I had with him growing up. We always had a great time together!

Good luck, hope you & DH can come to an agreement