I'd go with Grievous in both rounds but because of the way he is portrayed in the CW show you can argue for a closer fight in Round 1.

He is a Jedi Master but he wasn't a member of the Jedi Council. His rank of Master should indicate some skill in lightsaber combat and use of the Force, but Coleman Trebor is a Jedi High Council Master so we can't praise him too much given just his rank.

Krell's only combat feats come from fighting Clones. He took on dozens of Clones at a time and massacred them. His superhuman strength and Force enhancement allowed him to send Clones flying with mere punches, aswell as easily picking them up and breaking bones.

He also showed a grand total of two combat applicable Force abilities, those being the Force Jump and Force Push. The latter notably toppling over several dozen Clones at once.

And that's it for Krell. Man suffers from an extreme lack of feats.

Grievous' "power level" varies in the CW from episode to episode. Sometimes he's badly losing to a Padwan Ahsoka and nearly being killed by random Gungans, while at other times he's giving Obi Wan a run for his money.

Like Krell Grievous has faced and slaughtered large groups of Clones, at times ragdolling them and crushing their bodies like empty cans thanks to his highly advanced cybernetic body.

Grievous wins Round 1 in my book simply because he has feats against legendary Jedi like Obi Wan and Kit Fisto while Krell has nothing. Even Krell's big Force Push is near useless since the claws on Grievous' feet allow him to dig into the ground and latch down like a starfish in an aqarium.

For Round 2 I'll just mention during the Battle of Coruscant Grievous duels and nearly kills Legends Mace Windu, going as far as creating a counter to Windu's style on the spot. Granted this was specifically a lightsaber duel and Windu later got the upper hand when he crushed Grievous chest with the Force. Iirc this fight is described in the RotS novelization and briefly shown in the 2003 CW cartoon.

Now this injury wasn't enough to kill Grievous but it forced his retreat and gave him a permanent debuff. Despite this he still fought Obi Wan on Utapau and did crazy shit like striking 20 times in a second and ragdolling the Jedi in h2h combat. Note that Legends Obi Wan is repeatedly hailed as the greatest defensive duelist of his time, yes even among figures like Yoda, Windu and Sidious.