Stardust vs. Vivi Nova:
The stardust is vastly improved on the Ego Twist over a regular Ego and especially over the KGO (which it's almost incompatible with). Due to the airflow restriction, the inability to take them apart to clean, and the inability to rebuild them, the Stardust will never rank up to a Vivi Nova. Some people say the Vivi Nova has muted flavor but I think that what happens for me with my Stardust is that the center post gets so hot it's cooking my juice and on top of that it seems that flavor goes to the top you end up vaping that first and then you have this useless dark eliquid what used to be clear and tasty liquid.
The Vivi Nova is not by any means perfect but with ease of cleaning, the fact that you can rebuild them forever for like 20 bucks in wire and wick (do research before you go buying these though - youtube's Estarra is a good place to start), and the greater capacity makes it full of win. The drawbacks would be that it's ugly as sin, big and clunky which could make it harder to carry, and 3 times the price of a stardust. The last drawback is minor and cosmetic but you may want to get a 510 cone adapter for it so that you don't have this PV with a dip underneath the tank ruining the streamline of your ecig. I know both empiremods and Madvapes sell them. It's basically an adapter that fits over the top of your existing battery and then repeats the same thing with the two sets of threads (one inner, one outer) the juice catcher and the slots on top. Do count the slots on top in the adapter that you buy because some have different numbers than others and you'll want basically an exact clone of the ego-twists top. You will also want to clean your vivi novas before you use them to avoid what many complain about, that new atty taste. There are some threads floating around the ECF on different things you should do to improve on your vivi nova. Some people also say that you should ditch the extra wick that isn't even attached. Have fun!