Montessori vs k5... go! But actually what about skipping k5 and starting public school at grade 2 (unsure of what age that is for the non US school systems)
EDIT: I forgot daycare is prohibitively expensive. Whoops.
Apparently all US states require 6 year olds to enroll in first grade. (homeschooling might be an exception) IIRC some of the nordic countries start school at age 7, but they require you to already know how to read and write before enrolling.