Well, Oden similarly to Big Mom was portrayed as a natural born freak. Slapping the nurse against the walls as a toddler, killing a bear as a 5 year old child, taking a bath in boiling oil for an hour while holding the 9 scabbards above his head.
Hard to tell. ISDS seems to have more raw power, though Oden never got the chance to use Togen Totsuka on a mountain. But lethality doesn't necessarily equal DC, since Shishi Sonson is one of Zoro's strongest attacks and a finisher. Togen Totsuka is Oden's Shishi Sonson equivalent.
In terms of attack power/lethality, I think Togen Totsuka is stronger, since it's Oden's strongest attack. A mastered CoC Asura from healthy Zoro might be even above that though.