I'm not sure. There's three years between OH and his sister, and me and my sister, and I don't like that gap. It's too close, or too far, depending on how you look at it. It's too far for the kids to really be able to do the same stuff and play together properly, but too close for the younger one to understand this. My friend has 5 years between her and her sister, and that works much better. They have a proper "older sister, younger sister" relationship, without so much rivalry, since the younger sister understood she is indeed younger. Now they're both adult, they can socialise and so on together as adults and the age gap is irrelevent.
I think I'd like about 4 years between each. That does mean 12 years of toddlers, which is either a good thing or a bad thing, I'm not sure yet! My Mom reckons I should have them one a year, get it out the way. I think the problem with that is when they're, say, 5, 6 and 7, I'll be broody again!