K.K.
HI P.-
I'm not sure why your choices are Downey or El Segundo, perhaps your husband works in one place, and you live in another? Whichever, the most important thing you can do to decide where to send your little genius (none of us are biased ;-))is to VISIT lots of schools.
A great place to start is greatschools.net You can easily compare scores, parent ratings and facts about schools there. Then, you should call the schools directly (and soon) to find out when their Kindergarten Tea's are being held (most are this month!) Go see at least three schools - and see them each twice. The first time you see them should be an open house or Tea (when they expect you) and one surprise visit (when they don't).
I am the president of the education foundation in Westchester/Playa del Rey (wpef.org) - so, I can tell you volumes about schools just north of El Segundo. We started our foundation when the founders' kids were all in preschool and have been uber-volunteers ever since. So, once you've decided where to send your 5 year old - being involved is the best way to make sure that the school succeeds, that your child has a great school experience and perhaps most importantly that you can model for your child how important education and "giving back" are to you.
Good luck!