The answer to your question is quite subjective really. The “ Premium” as the name suggests is the higher grade of hair from a Simpsons perspective as it’s a lot softer in touch and feel on the face, with good backbone. The Sovereign has a more scritchy feel but not in a harsh unpleasant way. The premium being only available in the Chubby 2 size due to fibre size availability. The considerable price band difference supports this.
I have about 8/9 Premium in the Chubby 2 and about 4 in the Sovereign Fibre Chubby 3 all with various custom handles, are they great brushes ? yes they most definitely are. Are they worth the price ? In my honest option most definitely
if you go for the Chubby 3 in Sovereign don’t be put off by the daft lofts ..they do look a bit odd, but as @ Vates comments they do settle down after using for a while. Though I will ask Mark @ Simpsons why the shaping has gone a little bonkers .. as the T range are perfectly symmetrical? As are the Chubby 2 Premiums.
Hope this helps