The Sting 40 always feels very good (fantastic in fact), whereas the Venom 40 has felt under done (ie not powerful enough) each time I have used it. If trying to go for top speed off the wind, then the Venom would be my choice (and was when I got 36+ on the dial), but for general back and fourth sailing or pointing/racing, I'd use the Sting as it lifts and balances the board better. A larger Venom might do the trick (42cm), but I feel the Sting matches this board better and the Sting 40 goes well with the Reflex 7.8m. I have used the Venom 40 with my Reflex 6.2m (on the 107) and the fin felt good, so also a matter of matching the right fin to the right sail.
Figured out how to upload pic's. So here's a few more. The carbon one is an iS137.