Orthodontically speaking, what's the difference between a sippy cup spout and a bottle nipple?

I'm trying to get my son weaned from the bottle. He doesn't get one very often, usually 2, rarely 3 a day-for naptime and bedtime and occasionally just wanting to be cuddled and "nursed" (I practice "bottle nursing" recommended by Dr. Sears). He gets sippy cups for his meals and when he's thirsty. My husband isn't quite so convinced that our son needs to be weaned right away. He's only 14 months and still very much a baby in his (well our) eyes. I told him several reasons why it's good for him to be off the bottle and he pointed out that the sippy cup spout and a bottle nipple are basically the same shape, but the bottle nipple is actually softer than the sippy cup spout. Good point. So what's the big difference anyways? I am slowly introducing an open cup to my son, but it's a horrible mess every time. I know it's just water, but it's still all over the place even though there was hardly any water in it.