In-summary
AP vs AP DT Dante has 10x to Power and Speed due to hearts but DT Nero can keep up with SDT Dante, so DT Nero is probably still stronger and much faster than X10 DT Dante.
Speed vs Speed DT Nero should be much faster than even X10 DT Dante. DT Nero may be faster than SDT Dante, considering DT Nero could fly to between SDT Dante and SDT Vergil when they are about to clash. You can argue they would be slower due to exhaustion but the gap in speed here seems like it should still mean DT Nero would equate to or above SDT Dante's speed.
Regen vs Rege Nero should have equivalent stamina to DT Dante. DT Dante probably has better Regenerationn due to Healing Heart but it's questionable if SDT Dante would have the hearts, if he did this would be a blatant stomp. Nero should have a considerable advantage over SDT Dante in stamina but DT Dante has more Regenerationn than DT Nero.
Durability vs Durability DT Nero should have vastly superior durability to DT Dante but SDT Dante has heavily enhanced durability (may just be gameplay design but nothing contradicts it)
So in terms of stats it seems to go
AP - Likely SDT Dante
Speed - DT Nero
Regenerationn - DT Dante, DT Nero if Dante goes SDT
Durability - SDT Dante
Abilities/Hax
Summoned Sword - Equal. Nero can counter all of Dante's summoned swords with his own summoned swords, as well as his Devil Bringer. Dante seems to have superior skill/use out of his Summoned Swords however.
Time Manipulation - Based on what I have seen, Dante's only real time manipulation advantage is the type of manipulation that requires him to be doing combos (Chrono Heart) or specific attacks (SDT Time Manip). Quicksilver is questionable due to Nero having access to Ragtime, something utilised by Elder Geryon Knight so I think Nero's Ragtime should be equal to or superior to Quicksilver with non-breakage. Ragtime can work on Vergil, who should still have Mundus' power and thus the time resistance of Dante.
I may be wrong, but I'm pretty Quicksilver actually only has a specific radius so I think Nero and Dante are actually very equal in-terms of time manipulation overall. However, due to the rulings Nero has only one Ragtime to use so Nero has to use his Breakage VERY wisely.
I, from seeing videos of Quicksilver, also think that Quicksilver would still leave Nero as equal to or faster than DT Dante. Some argument can be made that Quicksilver SDT Dante may only really equalise the speed between DT Nero and SDT Dante.
So, overall I think the time manipulation isn't actually that great against Nero, seeing as it mostly slows time but Nero is so much faster that, while it weakens Nero, it wouldn't give Dante the advantage in speed.
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With everything argued so far, I think this comes down to two factors
- Bangle of Time vs Ragtime
- How DT Nero actually scales compared to X10 DT Dante and SDT Dante, in terms of speed and power
If Nero has no answers to Bangle of Time, Dante may stomp but if Nero can just use Ragtime from the start, Nero may win with high difficulty (due to having only one Ragtime and Dante's superior Regenerationn when in DT or superior durability when in SDT).