But even with this theory, Yamato has more powers than Rebellion even if wielded by a less powerful fighter.
So Yamato still is more powerful.
Just because it's capable of performing other abilities---i.e., sealing the Hell Gate---doesn't necessarily mean it's more powerful. Nero was wielding it, and he lost to Dante...but Vergil was wielding at as well---
Vergil, the person Yamato was literally intended for---in conjunction with Force Edge, and
he lost to Dante as well, who was canonically wielding Rebellion during that fight.
Honestly, which sword is more powerful on its own seems to be an irrelevant matter, since the Devil Arms seem to function at their full potential in the hands of the people they were forged for, or tied to in terms of significance.
My theory is that Devil Arms are more powerful in the bloodline of certain people---which is maybe why Dante threw Yamato to Nero during the confrontation with the Savior, instead of just using it himself. Perhaps Nero being Vergil's direct offspring made Yamato more compatible with his abilities, (which is maybe why Dante can't use it to its full potential, and tossed it to Nero in the first place).
Of course, this is all head-canon. This
is Devil May Cry, after all...the franchise whose lore that's had all the revelation and expansion of a mole-hill.