Let's start first with a comparison with what you have now. Toll Brothers has built high end in San Ramon ... average list and sale price since 1/1 is above $1.6 million ... homes are bigger than you have currently. Average sale prices in Walnut Creek and San Ramon overall are currently running at $815,000 to $860,000. In general, you get newer in San Ramon, mostly 2 story homes, often on smaller parcels, almost uniformly in larger subdivisions than in Walnut Creek. If you rely on school test scores San Ramon and Walnut Creek are generally comparable
Lafayette is older, more hills, more single story homes and has an average sales price currently of about $1.35 million... for a smaller home in many cases. Again, if test scores are important to you, then Lafayette will have the edge. Bear in mind that to a substantial degree test scores follow the socio-economic level of the community ... the facts support the notion that higher income communities support personal educational excellence to a greater extent than in other communities.
Right now the average price of a home throughout the region are climbing faster than I have seen it in my 30+ years selling homes in Contra Costa County... approaching 3% per month. So, if you rent for 6 months expect to pay 15% to 20% more than if you bought now.
As to private schools, expect to pay $2000 to $2500 / mo per child for a top quality, non-religious based private school (eg Bentley School, Lafayette).
You need the expertise to help you make sense of all the different choices that you have. As a life-long resident of the East Bay, most of it here in the Central County, I may be able to help. Let me know.