Is it an insurance thing that's making you have to choose? Most higher-risk moms I know use both the NT test and the MaterniT21 test if they're not doing the CVS. Honestly, I would still push to get both if I were you. It really helps to get two different tests for peace of mind. The MaterniT21 apparently doesn't charge you more than $235 if your insurance declines coverage- just a thought.
I posted a link to an article in the Washington Post about these tests earlier in this thread. They are not covered by insurance yet as they are considered "experimental" for the moment. The article is really worth a read as it gives some good context for these tests and the huge demand for them since they are simple blood tests.