There are pros and cons to any arrangement. Closer together and they have more potential to be playmates and close friends. They might be able to share more things (rooms, clothes, birthday parties, etc) which might be nice. But there might also be more competition. And there might be fewer resources. But then, the parents get out of the diapers faster, potty training faster, etc.
Personally, I think 4-5 years between the kids is ideal FOR US. The way we see it is that it gives both children the chance for undivided parental attention during infancy and toddlerhood. The oldest goes off to preschool/kindergarten and so there's plenty of time and energy for the second child. When they're spaced closer together, the parental attention is more divided.
HOWEVER, with adoption there's more variability. You have the option to adopt out of birth order. You can also "twin" a child when he/she is 5 or 10 or whatever, etc. "Bam! You have a new sister. And she's the same age as you!" How cool is that? Or sometimes things just happen, like when the birthmom gets pregnant again and she or the agency look to you to adopt/foster.
Plus with adoption there are other issues, as the OP mentioned. The process may go more smoothly or be easier to tolerate when the first child is a certain age than when he/she is another age. For us, we felt it was easiest to go through the process again when our first was school-age. It would give us more time to explore the options. And so that's where we are now. We're gearing up to adopt a second. But like the OP we're not interested in dealing with the state again. Our experience was so terribly negative that my husband simply refuses to do it again. This time we will adopt from a private organization.
Queenjane is right about holiday placements. However, accepting a holiday foster placement is no guarantee that you'll be able to adopt that placement. And if dealing with the system is difficult for you, I wouldn't recommend inviting more of that stress in at a time of year that is already stressful.