I've use the Avantis before, but never the Volos. I'll put my input in based on what I can tell about the fin by sight.
The Volo's are a ton more flexible than the stiff Avanti's. What this means is that it'll feel easier on your legs when you kick; rather than pushing through the water you're relying more on the recoil of the fin to give you power. The Avantis work more or less in the same way, but since they're stiffer you may be able to get a bigger burst of speed out of them provided you have the strength to do it.
With the Avanti's, since they're stiffer and flex less you can get more minute control. In a horizontal hover position, you will find it easier to do helicopter turns, back kicks, and frog kicks with out heavy foot tweaking. (this is assuming a good kicking technique is used though). You'll also be able to get more positive feed back when doing it. Doing these specialty kicks with flexible fins is a challenge because there's usually a delay in you kick and the movement you receive from it.
Recommendation
If you're planning on doing photography or poking your head into tight spaces where you need stationary mobility, then go with the Avantis. If you're planning on diving in wide open expanses and cruising through most of your dive (nearly non-stop swimming) then I would recommend the Volos.
I will say that the Avantis feel long on you when compared to Jet fins or flexible paddle fins. If it's your first set of fins though then you'll not notice the difference unless you switch. You can live with them and not notice or not care. I know plenty of divers who do. I'm just used to shorter fins.