Grand Fleet captains are stronger, generally speaking.
They all scale to a level on pre G4 Dresrossa Luffy. Cavendish and Barto are basically Supernova themselves. Pre G4 Dresrossa Luffy was definitely in Tobiroppo/Yonko-executive tier.
General power levels:
Habuka > Barto >= Sai ~ Franky > Robin >= Brook
And, we don't even know how strong the GrandFleet captains are now, maybe they grew much stronger like SHs did as well.
The only round I see mid SHs winning are maybe Robin vs Sai, Franky vs Sai and Robin vs Barto.
- Barto, because Robin can sprout arms inside Barto's shield (well, might backfire if Barto decides to form a shield a bit closer and cut off the sprout arms)
- Sai vs Robin can go either way, maybe Sai is physically strong enough to break free, his strength scales to G3 Dresrossa Luffy
GrandFleet captains win every other round, as well as the team matchup
The only reason Robin did well against Cavendish was because he had to cover an entire mountain to reach where Robin was. If the fight starts within say 30m or so (say Colosseum), Robin would get blitzed.
Hakuba blitzed and one shot every single character in the Colosseum. blitzed and one shot Doffy's executive (someone who easily defeated Bellamy), blitzed Barto (he couldn't follow his speed and survived just by Barrier). Even someone as strong as Sabo, was amazed at how fast Hakuba was.