You may have already gathered most of the comments here from Yumbayumba1 are complete rubbish if not outdated. Just a few points:-
'Most' of the schools in Letchworth are not appalling at all, inc the two private but overall Hitchin does have better secondary at least due to Hitchin girls and boys
If council estates concern you, which is crazy, funnily enough those in Hitchin seems to have been missed
Lots of shops in Hitchin have closed down, especially of late. As for the pubs, many of the old historical ones in Hitchin are now closed, whereas in Letchworth three have opened. Lethworth also has a farmers market, but does not have the large market Hitchin has (be it the council have tried to reduce it a few times)
Letchworth has an amazing cinema and many leisure facilitates. It has a greenway cycle path, kids farm.
It is generaly more green and as many amazing houses should you be able to afford.
The train station is in the centre of the town, not on the outskirts like hitchin. And direct trains also into London Kings X
We've lived in both towns, both are great places to live.