Look, it comes down to practicality in the end.
500 class bikes have similar potential, based on displacement, and the difference would be rpm potential, limited mostly by stroke length and breathing capacity. The twin has some advantage in lighter pistons, but a disadvantage in smaller valve area.
We have major amounts of experience in making these singles very high performance, and the Bullet Whisperer race bike will show tremendous capability for power and rpm in the 500 single.
We also are gaining experience on the twins because we're doing the heads for Scottie's Bonneville 736 Chief twin project. We have developed roller rockers which should work in any of the twins, but we haven't yet verified that to be true for the 500 twin. We can make power in the twins if you want it.
The issue becomes cost. We can do a whole lot more work on one head, for similar or less money than could be done on two heads. So, for a 500 class machine, I consider the choice is clear for the single.
It's simple economics, and if we can give similar power from the single for less money, that pretty much sums it up.
If you want to do a bigger displacement twin like Scottie is doing, such as an Interceptor engine, and build that up, then there is more power available from the larger displacement twin in that case, and we can use our roller rockers in that, and Scottie can get you the steel con-rods, and such. But it's a lot more money, so you have to wrap your wallet around that before you get started.