It depends
NHS advantages: really useful to find out how things run at the local hospital, and to get a tour of the wards. You may get a better flavour of 'how things are done', including pain relief etc. You may meet some great new friends. Free of charge.
NHS disadvantages: variable quality of group leaders (antenatal classes may be an unwelcome task foisted on reluctant midwives), group may be big and so not so much time for asking questions and talking.
NCT advantages: many women go because they want to make their new mummy friends, and I found it invaluable for that (we still meet up 5 years later). There's some time for relaxed discussion. You may get more discussion of how to do things your way, not the hospital way.
NCT disadvantages: it costs - quite a lot. Some women find it too doctrinaire, too pushing an agenda (I didn't, but I guess it depends on the quality of the teacher).
It really comes down to how lucky you are with both the teacher and the other women. But you could try both - lots of women do.
Best of luck to you. First pregnancy is so exciting - I envy you!