How about a Mercury vs Toon City vs RDK vs Sugarcube when comparing to Star Butterfly and Marco Diaz?

It’s hard to compile proper screenshots to exemplify everything, and some differences can be understood only by watching whole scenes / episodes, so I’m mostly going to be descriptive

Mercury: thickest outlines out of any studio, consistent models and sizes, but with a lot of bounciness and deformation during weird faces / action moments. This studio put the most emphasis on teeth, eyelids (especially for upset expressions), and especially eyebrows, giving everyone a less soft vibe than later seasons, on average. Overall they obviously followed character models, but rather than trying to mimic the style of the boarders, they had their own “style” (other shows made by Mercury share similarities). Shorter necks, and more details for weird faces, especially for Marco. Not “gross out realistic Spongebob close up” tier, but still something rarely seen in later seasons. Maybe a bit more “cartoon from the late ‘00s” style than “cartoons from the ‘10s” style.

They worked with “puppets” animated in flash, so there’s some tweening here and there, but barely visible. They were that good.

Toon City: hard to point at specific characteristics, like RDK this studio followed the style of the individual storyboarders of the episode, but with a strong tendency for going off model or having weird proportions. If I were to make a super generic comment on this studio, I’d say that the characters - or at least Star and Marco - looked shorter in many of their episodes, but even that wasn’t really that consistent.

They worked with vectors, but without tweening, animating every individual frame. 

RDK: my favorite, almost impossible to pinpoint general characteristics here since this is the studio that followed the boarders’ style the most, but the first word that comes to my mind is soft. Everyone looked cuter in RDK episodes, and while I did obviously miss the bounciness and action packed scenes Mercury had (and RDK did suffer from “cheap” looking animation more than other studios), I think they drew the characters at their best. Especially Marco, who has slightly rounded and bigger eyes and smaller eyebrows in most RDK episodes, giving off a stark contrast compared to the almost smug/snarky feeling expression her had in Mercury episodes when idle.

They draw traditionally, with ink on paper, to then scan the frames and color & animated & composite the whole thing digitally, so you can see some imperfections in the outlines, like where the pen left the paper, or where they probably inked the same section multiple times, leading to a line of variable thickness. 

Sugarcube: a weird studio. When they go offmodel or have to animate special or unconventional poses, they easily rank second best for me; but when the characters are idle or just standing there talking, they often look too stiff - especially Star and Marco, with way too “regular” and geometric eyes and shapes, small pupils and irises, and longer than average necks. It improved a lot during S4, but I still wish they’d have made them eyes a bit bigger.