Sonic's sprites in Sonic 3D be like:
Mega Drive: Utter f*cking garbage that looks like shit with legs
Saturn: Clean, beautiful and renders of our colorful beloved character, Sonic The Hedgehog

I noticed when counting the amount of frames when comparing this to the Genesis version, that the Saturn version has less frames on a few animations than the Genesis version. Specifically on the wait, spring, and ring hold animations.

With pre-rendered graphics, they would have taken the time to fine tune the animation and probably hand fix any problems (like the menu screen). They would have taken a long time to render back then but the alternative would have required a team to do sprites that may have inconsistencies (that some games did do). It's like why pre-rendered cutscenes generally look better than the in game models (at least character models, lighting and smoothness wise, a lot of pre-rendered cutscenes have however poorly aged due to the design of the models, the textures, blurring/interlacing or the resolution/frame rate is just too low ironically making the in game graphics look better...).

As @UnknownUney says, its poly count is just too high and with the funky quad setup on the Saturn, it wouldn't have been possible. Even the Dreamcast probably would have struggled...

Too high of a poly count for the sega saturn

Huh, you know I would have figured they would have used the 3D model that they digitized for the Genesis by itself given this is the Saturn.

oh yes
is that the one sonic presented on the very early demo from 3d blast published by jon burton himself?

Oh yeah my bad, it was released on saturn aswell but for some reason was also pixelated

3D Blast was made around the 3D BOOM Era, where everybody was extremely interested in that type of thing, and pixel games started to recede in popularity, it was almost necessary for game producers to go over to 3D and capitalize on the sucess they can gather from this, rather than making 2D games that will get much less sales. Unfortunately, Sega was making the new Sonic game on the.. genesis? for some reason, but due to the genesis being unable to make polygons and what not, they had to go for 2D. But, they had an Idea. Making the game isometric / at a 3D Point Of View could gather the game some popularity and sucess, but the only way to make the animations were to render them in a 3D Modeling software, and convert it to the genesis. ( They had to render EVERY single frame. Which is why there isn't as much sprites. ) But this posed a problem. Since the genesis had a limited color Pallete and resolution, this resulted in sonics arm being invisible some times due to the resolution and sometimes Sonic's eyebrows appeared as his eyes color, or his thingy in the middle of his eyes appeared as his eyes. Also, rendering it frame by frame made the animation more and more linear and un natural, as I'd assume making so many individual poses would be a hassle, and rendering would take more that todays usual because of unadvanced PC Hardware and specs. Ultimately it wouldve been a better descision to simply render just have the 3D Model in game but they were on the genesis so that couldnt happen.

He lookin’ good.

That’s a nice blue he has.

Nice job! After over three years, he finally got ripped!