S.P.
I think it all depends what you want for your family and your kid. Definitely look around at all of the options. The school we ended up at was never on our radar. I have a kindergartener and a 2nd grader in Seattle Public Schools. The cost for full day K this year is $237. They say you can opt to do 1/2 day but they do not accomodate the 1/2 day kids. Your kid would miss math or writing or reading if he missed the second half of the day. Also, with sSPS, you need to go to your neighborhood school. The Northend schools are all quite full. You would likely end up in a large class. My daughter's K has 27 kids. Eeek! It is really difficult to get into the option schools (if you have twins you can run the possibility of getting one in but not the other). Also, if you get into a school other than your neighborhood school you have no guarantees that your next child will get into the same school. We had that issue last year and our daughter didn't get into our son's school until the day before school started (and they changed the rules after my son had started). At this point you know the rules and really don't have a leg to stand on. My point being if you want your children to all go to the same school go neighborhood school or private school.
There are great private schools out there and you should check them out. They may or may not work for your family for various reasons. There is definitely something to be said about smaller classes.
I don't know much about the Shoreline district but I think they have a good reputation. I have a couple of friends who are happy with their kid's education. Shoreline's gradeschools also go until 6th grade which I think is a selling point.
Remember with public school you will get kids and families from all walks of life. Every year my kids have been in school they have had at least one if not two very disruptive kids in their classroom. I look at it as our diversity in our NW Seattle neighborhood but the teachers easily spend 1/4 of their time working with those individual children.
I think it will be hard to go wrong as long as you are involved in your child's education. Make sure you take advantage of as many volunteer activities as you can. Watch your child play and who they play with.
It's pretty amazing to watch your children grow. This year I officially went out of the preschool business. So strange but not any less magical and exhausting.
Good luck with your decision. I know you will make the best one for your family. Remember you are not stuck there are lots of options.