I chose a termination after a positive DS test result. I was young, I wasn't ready for children, and definitely wasn't capable of raising a child with additional needs to the level they deserved. It was the right decision for me, though it was very hard. I have had "what if" moments, I have seen children who look like mine would have, and I have never regretted my choice.
Whatever decision you make, it needs to be what is best for you. If you're ready to take the leap and raise a child who will need more care, love and attention than you had planned- you will do it superbly. There is support for parents of disabled children, and you can get things lined up far in advance.
If you choose the other option, you're making a choice that you know is fair for all involved. It can be a kindness to prevent suffering- for you, for your career, your family.
Whatever choice you make, however you go about it, and whatever your reasoning is, you will not make a mistake. There is no right or wrong choice. Take some time, ask questions, speak to charities and support groups and medical professionals. Make a decision based on balanced research, and you cannot go wrong.
I'm thinking of you, and sending very best wishes.