This boils down to Feats. Remember since no PIS and no CIS, Scratch and Grounder don't ruin Robotnik's plans but Sonic doesn't easily fall for them either; so many Robotnik's schemes may either work out perfectly or break down.
Sonic has a lot higher regeneration than Robotnik, so they're both hard to take down with brute force.
Robotnik however, has lots of machines that bypass conventional durability. E.G. his Robot Transmogrifier Ray essentially one-shots anyone it hits - while Sonic has no means to permanently OHKO Robotnik. Robotnik's range is also higher, and Robotnik's intellect is higher (at least, when it comes to scientific disciplines).
They both have time travel, but since Robotnik has preparation time while Sonic doesn't, Robotnik can time-travel first, and therefore start whenever and wherever he wants while Sonic has to get there first.
Robotnik can time-travel to "before the battle starts", to place where opponent starts battle in (Sonic in Tails's house in our case), and place traps there. Including placing the Force-Field Generator exactly where Sonic would start, so he instantly gets trapped and Incapped the instant battle proper actually starts.
Robotnik can also time-travel to Ancient Egypt era, where Sonic's ancestor lives. If Robotnik messes with Sonic's ancestor, Sonic gets erased from existence (like it happened in one of episodes); and if Robotnik outright kills or roboticizes Sonic's ancestor, then adding Sonic back into existence becomes impossible. Since Robotnik can position himself in firing range before battle starts, he could shoot Sonic's ancestor the moment battle starts - therefore erasing Sonic from existence before he can intervene.
Robotnik and Sonic have same speed (both due to equalization, and actual speeds of base forms), but Robotnik has ranged weapon capable of one-shotting Sonic while Sonic would need a lot of hits since Robotnik has all Chaos Emeralds, including Invulnerability and Immortality. Robotnik can get even faster with Speedemint Gum. Since they both have all Chaos Emeralds, they're both Invisible - but Super Robotnik has super-senses to detect Sonic and see through his disguises, while Sonic has no means to detect invisible Robotnik.
Robotnik just has more Hax number-wise than Sonic has.
Robotnik also has henchmen. Not just Scratch, Grounder and Coconuts - all robots he ever built. Many are in same Tier as Sonic and Robotnik, but while they regenerate easily, they can't easily kill Sonic either due to regeneration. Nevertheless, since PIS and CIS are not in play, robots would help Robotnik more than they would hinder him; at worst, they could stay at home.
Overall, it seems that Robotnik has the edge. So i vote Robotnik.
