Nova Thread Lifts vs Silhouette InstaLift. What is the difference between them?

PDO and Instalift are quite different and have different functions in non-surgical rejuvenation of the face.  PDO threads which are made of Polydioxanone have great collagen stimulation effects and therefore good for increasing volume and tightening of the skin.  Instalift which is made of poly-L-Lactic Acid (PLLA) is stronger but does not have as much collagen stimulation effects.  However, Instalift threads have "bells" incorporated into the structure of the threads and have great "pulling" effects.  Therefore, in order to have good pulling and tightening of the facial skin, as well as increase in volume of the face, it is best to use both PDO and PLLA for non-surgical facelifting. 
In addition to tightening of the skin and V-shaping of the face (if so desired) the quality of the skin also improve with these treatments.  The skin will appear smoother and tighter and the pores will be less noticeable.
Another good ancillary treatment is subcisional Radio-Frequency treatment, in which RF is applied underneath the skin using a need inserted under the skin and produce RF to tighten the skin.
Consult a plastic surgeon who does all these and he can tell you what is best for you and can give you the best result.
Best wishes.

The Nova threads and Silhouette threads are made from the same material but have different mechanisms of insertion.  Most if not all newer threads are made of PDO, a dissolvable biocompatible material that is made from the same material as commonly used surgical sutures.  The longevity and efficacy of the threads are the limitations of the procedure and you should do your homework prior to having the procedure so that your expectations are met.