I have googled the nutri bullet as I didn't know much about it. A friend has one and he uses to make his morning smoothies with nuts, fruit and veggies.
I think there are quite a lot of differences from what I can gather. The first and obvious is the price. One is a lot cheaper than the other as the thermomix is almost $2000 Australian dollars.
Does the Nutri bullet cook as well as blend? Does it have the possibility of steaming food as the thermomix does? Does it have scales?
Perhaps if you told us what you want from the machine that you buy we can help a little more. Have you looked on the thermomix site?