very different.
Ayahuasca is the vine, and it contains no DMT at all. The DMT is found in the admixture plants and they are added to potentiate the visions of the vine. The harmala alkaloids found in the ayahuasca vine are what make "ayahuasca" ayahuasca.
Smoking DMT pales in comparison to ayahuasca IMO. Ayahuasca is deeper and more meaningful and more healing. It can grab ahold of you if you take enough admixture though and not let go for some time, where as smoking the DMT just lasts a few minutes.
DMT is amazing no matter how you take it..but personally I think ayahuasca is a superior experience in a number of ways.
Long live the unwoke.