Maybe it's just me but Hopeless Soul seems to have way better hax to use in a battle than Chara had before his/her absolute state, while fighting Sans. I mean, yeah, (s)he was strong, but Hax wise Sans had much more going for him (the main thing that screwed Sans was "Whatever, I can repeat this until I'm able to aim dodge everything you throw at me").
Also, I don't know how fast Sans is, but I'm preeeetty sure he's not fast enough to be keeping up with an Omnipresent being - Chara only gets their Omnipresent rating after reaching their Absolute, and Absolute Chara would murder stomp Sans, so they don't count.
I'm leaning much more on Hopeless Soul for this.