mine are 10 and 11 (13 months apart, a boy (older) and a girl). my relationship with my siblings has always been either negative or non-existent. as a result, i always, always said i only ever wanted one. but after my son was born we went to visit my husband's family. as i watched my husband with his brother talking about when they were kids, i knew right then and there that i wanted a second one ~ someone my son could not only spend but look back on his childhood with. before i knew it i was pregnant again, and my son was barely up on his feet when the second one came along. so, just like twins, these two have never really known life without each other, and they are extremely bonded to one another.
i definitely, definitely recommend two, and the closer in age the better. not only will they be more bonded to one another, but you will be able to resume the other parts of your life sooner, and you won't be running in two directions for the whole time they're in your house. they can enjoy a lot of the same things and keep a similar schedule if they are developmentally close to one another.