I'm gonna go with the Numero Cuatro Espada here in BOTH scenarios.
First off, if I'm not mistaken, I doubt that Riku is able to handle Sonido. If he can, what about Ulquiorra's strength? It made Ichigo in Bankai + Hollow mode look like a lil bitch. Scenario 1 is taken care of here.
Secondly, can Riku handle Cero/Gran Rey Cero/Cero Oscuras? I doubt it. Correct me if I'm wrong though. Scenario 1 is also taken care of here.
Thirdly, Ulquiorra has TWO releases. His final release was nothing short of fearsome. Ichigo had to actually DIE to defeat him. So, Riku has to have an ability equivalent to Ichigo's Berserk Hollow form in order to defeat Ulquiorra. As of now, I know Riku has Darkness as part of his abilities but I'm skeptical as to whether it would do much damage to the enemy. My worry applies to the Key Blade as well.
Scenario 2 is also in the favor of Ulquiorra here.
Let me know if I made any mistakes in my argument. Thanks for reading.
We know two things, Ulquiorra's lance is less destructive than final getsuga's hill buster, and that Riku has sliced up skyscrapers in KH2.
Look at the 43 second mark.
Riku is capable of high speed teleportation as shown by Dark Aura.
Riku has also shown that he can fight someone who manipulates time, although it is uncertain as to whether or not that was possible only through Mickey's help or not.
Plus the whole laser deflection bit at the final boss of KH2 makes it look like Riku is incredibly fast.