This is very helpful advice @d_e especially since you have, and like both the Torga C3 and Duke 2 as well as the synthetics I'm considering. I'm inclined to take your advice to get the Semgogue first and then the Duke in the future. Of course that will mean violating my "one brush" rule. But who am I kidding?I too prefer brushes in the 20 -22mm range and suggest since you know what you like stay within that range. I have both the Torga C3 in Finest and the Duke 2 in Best. Picking one would be tough. If I had to I would get the Semogue first and then eventually the Duke 2. Both are worth having and deliver a fine shave. My Torga has a lovely face feel with some scrub in the background, the Duke is a bit softer on the face but with good backbone and tip feel.
I also have an Eagle 2 in Sovereign fibre and had a Muhle STF 21mm. Both are lovely brushes for synthetics and I would be happy with either. If required to pick one the edge would go to the Muhle.
As I posted previously, I found a 21mm Muhle STF for $31 at YourShaving.com and they also have a good price on the Torga C3 ($60). This means I could score two solid brushes that are quite different for what it would cost for a Duke 2. Perhaps that future Duke could even be a 3 in two-band, to kick it up a notch. Who knows? I'd have to start saving up though cuz paying north of $200 for a two-band Duke 3 per @Barbarian80 and @CJB3 recommendation (thanks fellas), would put me in a league I had not expected to be in!
Cheers and thanks again for weighing in,