I'm assuming you have homed in on those because of the proximity of the A1 and/or rail line into London?
From that list, probably Letchworth. Personally I'd avoid Sandy and Arlesey, and I don't know St Neots very well.
Have a look at Langford, Henlow, Clifton, Stotfold, Fairfield (which is basically a newly-built village in between Stotfold and Letchworth) and Shefford. All small towns and villages, and only a couple of miles away from the places you've mentioned already, and apart from Fairfield have older houses as well. You need to move fast, prices are rocketing round here, it has suddenly become part of commuter-land. Arlesey is the best place to catch the train, huge car park and reasonably priced.