For what it's worth, I think they will throw everything at the new school to ensure it will succeed.
The new KWS Head is currently Deputy Head at RPS, the new Head of English is part of the SJL SLT, the new Head of Science is the SJL Assistant Head of Science, the Trustees include the Heads of RPS, SJL and St Georges, and there is partnership with University of Hertfordshire and Rothamsted Research.
My son will be going to KWS as one of the 'founding' pupils and I have mixed feelings about it as someone born and bred in Harpenden - it's quite galling that my children are unable to get places in existing schools in the town in which they live. However, that aside, I know of several people who have had the choice to send their child to RPS or SJL who have actively chosen KWS instead.
You will find many people in Harpenden opposed to the site of the school, and the building work that is currently going on (the school won't be ready for the first Y7 cohort until 30 September, mainly because of the delays due to various legal challenges the opposers had to the location), but I suspect most of those people weren't in the position I and many others were of no other school place being offered.
We shall see what the future brings, but so far the Head is saying and doing all of the right things, and I think my son will benefit from being one of 180 rather than one of 1200 or more.
You could look at buying somewhere close to both SJL and KWS which would give you a better chance of a choice between the two when you need to make the decision, but with the continued development of what seems like every spare scrap of land into more housing, there's no guarantee that in 2 or 3 years you'd get into SJL unless you actually live within a very small radius.