Here's my experience:
My nephew and son share a birthday and they love it! When they were younger (they are now 9), we usually did parties together. As they've gotten older, they each like different things, so we don't do that anymore. But they never seem to act like they feel something is being taken away from them by sharing a birthday.
My sister and I also have birthdays one day apart, and we usually celebrated together even though she was 9 years older than me. Never bothered us.
That being said, I think if you have a choice I would make them different days. But be aware, that whether they are the same day or just a couple days apart, I think people will be lump together no matter what you do.