The decision to test or to not test at all is totally up to you, but I suggest consider what testing means and what you would do with either outcome before you decide to test or not to test. There is honestly no right or wrong answer, just whatever is best for you.
Secondly, check with your insurance company first before you decide on either test to see if they are covered, and if they are how much they are covered. Some insurance companies do not cover the blood test that you discussed as an alternative to the NT scan because it is still in the experimental stages.