I have chosen to have the nt scan, for me purely because it can give additional information for anything that may arise at the 20 week scan, although with dd they did this at 20 weeks whereas this time they're doing it at 12 weeks.
For me, if I get high risk of downes then that's not something I'm too phased by and I wouldn't opt for the amino test as I'm not keen on invasive testing for the baby, whereas a scan and blood test (only invasive to me) aren't really here nor there for me. As others have said, it doesn't mean that the child will definitely be born with downes.
As I say, for me it's a collection of information for the drs for my baby and so, purely for me, I'd like to be able to give them as much information as possible in case something should happen.
As others have said, these aren't always fully accurate, the same as growth scans, but the chance is still there.
Did you get a leaflet from the midwife? I had a booklet given which discusses it all to help make an informed decision that is best for you, it was really good. I imagine it's online on the nhs site too xx