Hitchin - hands down. I live here so I am biased but I always say Hitchin would be perfect if it just had a beach. People are really friendly and it has a great chilled, laid back vibe despite it being quite a busy little town. It feels a very creative place too, but maybe that's the people I know.
It is a lovely, green place - I mean there's a great big hill right in the middle of it with small woods you can potter around. Plus lots of fabulous country walks on your doorstep: you can walk to Priory Fields and Oughtenhead Nature reserve, easy car ride out to Pegsdon Hills. I have a large hound and walk the dog every day somewhere different, everywhere but Pegsdon Hills is within walking distance of my house.
The town centre is full of independent shops, yet surrounded by supermarkets so within five minutes there's a Waitrose, Sainsbury's and Asda with a very handy Tesco Metro right next to the station. Lots of nice restaurants too with a whole variety of types: Turkish, Japanese, Fish, Pizza, Tapas, Mediterranean, Chinese, burgers, bbq... tons of them. Some really nice independent cafes and loads of lovely pubs: I love the Vic, the Highlander and the Coopers. There's a market which has seen better days but still has some great traders. The plant stands are brilliant and very cheap, the material stand run by Dawn at the Church side of the market has absolute bargain fabrics. My dog has many friends in the market and would probably rather go there everyday as he always gets tons of treats.
The Churchyard with river makes a nice picturesque centre (watch out for Hitchin legend Grumpy the goose) and I always think Hitchin is particularly lovely in snow or at Christmastime with all the pretty medieval buildings.
The theatre is a good plus for the town and you can see live music in a few different venues, although there's a real worry that we'll lose Club 85 which has been a major deal in launching lots of local bands and has a very eclectic selection of live music.
There are a few gyms - Archers with the swimming pool has the most options. My husband goes to the Exchange but I don't rate it. There's a great yoga centre near the station and Yoga Mind Space does excellent yoga classes. Lots of things for kids to do too, although mine are much older now so I'm out of the loop with that one. However one thing that I do notice having older teens is that it is very easy for them to go out safely in town - lots of places for them to socialise and an easy, safe walk home. Same for me too I guess. Also nice easy walks to their secondary schools. In fact walking is the key thing here - we walked to their primary, I walk to the station, I rarely use the car for anything other than long journeys or to find new places to walk the dog at the weekend: Barton Hills is a new, wonderful find.
I personally can't fault the schools as both of mine did very well and both rated their schools highly. The secondary schools are notably good.
Don't know about choirs or orchestras though...
I personally think Hitchin has more soul, especially the centre, but you'll certainly get more for your money in Letchworth. Watch where you buy as I think there has been planning agreed for hundreds of new homes in both places.