MrsMillions - I'd do the later NCT course because if you have the baby say 6 weeks before everyone else on your course, you'll find they're still on maternity leave & then it'll probably be a few weeks after their babes are born before they want to meet, so your baby could be a couple of months old.
However, you could contact your local Nct and see what people locally usually do. Maybe you could join the "right" course but hop onto the breastfeeding session of a later/earlier course?
I found it really helped to have people going through the same thing at the same time - and in the early days a few weeks seems to make a big difference. Having said that, by 6 months it won't, but those early weeks are a key time for having support & company.