Johnny has, really good pain tolerance, way more versatile, and teleportation from what I can see. His actual moveset isn't really impressive when you compare it to Spinel's regeneration (and vice versa really). But There isn't really much in his moveset that'll be applicable in this fight. Like- Hurricane Hands and Power Poots are the only actual offensive moves I can see on his profile, so in terms of versatility, it's not really much. It'll surprise Spinel for a bit. But it isn't nearly enough to give Johnny the win. Once she realizes that those are the only two offensive abilities he has, with her dura advantage, body control, and regen he will literally not be a problem for her. The only issue I can see is his teleportation that he does in character. And his pain tolerance of course. But there's no way he's putting her out. Even if we assume that it's a battle of attrition/stamina, Spinel should take it.
Spinel has my vote, easy-mid diff.