I've lived in Hitchin and surrounding villages for 30 years.
There is no aircraft noise- we are in Pirton and nearer Luton but apart from being able to see Easyjet planes in the sky we don't hear any planes as they are too high in the sky!
Letchworth is IMo soulless- sorry if anyone lives there and loves it. It is a 'new' town and has no real character and the shopping facilities are tired to put it kindly.
Letchworth has a lot of expensive houses ( talking £1m+) which are lovely, but so does the better parts of Hitchin.
Hitchin is more upmarket, and has a more cosmopolitan feel- cafe culture, nicer shops, lovely Waitrose, but also cheaper supermarkets too including an Asda.
There is no cinema but you can get to the Leisure Park in Stevenage easily for that. Hitchin does have a small theatre and has a music festival every summer- Rhythms of the World.
There are nurseries in Hitchin and 2 single sex schools at secondary age both of which are oversubscribed. They are better than the Letchworth secs.
The best areas for property and schools are the Wymondley Road area where you are in catchment for Willian Ransom, and St Andrews in Benslow area. Mary Exton JMI is quite good- Oughtonhead and Wilshire Dacre are not.
Generally, the south side is considered better than the north, with the west side being 2nd best- Samuel Lucas school is ok.
Anything else, just ask!