[heads-up: this page is badly written and all the images are all stretched but whaddaya gonna do]
Prototype imagery on the left, final revision on the right.
While the final version includes one of those "export this game outside Japan and YOU'RE DEAD BUSTER" screen and a look at the top 15 high scores before the SEGA logo, the prototype cuts the crap and starts right there. The final adds "©SEGA 1992" to the attract mode.
The emblem remains the same between both version, but that's about it - there's a background, Ray and Mighty thrown in, and Sonic's head is redrawn to look more rounded.
The prototype relegates characters to what player you are. The final throws in a character select once you start, and also when you continue from dying.
For gameplay, a number is added beneath the life bar to show the rings you've collected. Also, the text I can't read in that window is slightly different. And if you want to get really nitpicky, the screen's positioning is just a liiiitle bit different between the two.
The prototype jumping isn't really suited as anything more than an attack on the very few enemies in the game, and the how-to-play sequence doesn't even bother showing that, so it gets a much shorter message. The final gets in some box leaping and wall busting to demonstrate the improved height. That wall there is also the only time it shows up in the final!
The characters in this sequence are all redrawn for the final, made more gangly and a little less frighteningly proportioned. It still uses the original proto sprite when being hauled back to the tower, though.
Naturally, the final version features more flair regarding the end of a level. The prototype focuses on the end of the level, zooming out to show Robotnik expressing his outrage before moving on to the world map - the final is all "[stage name] COMPLETED!" and tallying upyour rings before it just fades to the world map.
The prototype features only five of the levels, versus the final's seven, and goes through them in a different order.
Volcanic Vault - Trap Tower - Landslide Zone - Ice World - Desert Zone
Volcanic Vault - Icy Isle - Desert Dodge - Trap Tower - Landslide Limbo
A few names got changed, as you can see. Wild Water Way and Eggman's Tower are completely unseen, and according to my very basic and crappy value editing, they can't be accessed at all in the proto.
Your dudes land in a completely different area in both versions, the final launching you onto a slope with boulders rolling after, while the prototype is just a rather mundane tunnel with fire bursting up from beneath the ground. The final has this as the near-final segment of its course, featuring a different wall and sparkling protrusions from the walls that nest rings.
The final ends once you've reached the bottom of this path, but the prototype continues onward with some conveyor belt and flamethrower action.
The area is positioned before the previous area in the final, after the boulder chase, and it doesn't feature the flamethrowers at all, opting only for the pits of rising fire. The prototype, in turn, has none of those here. How about that!
The prototype's exit is where the bridge would begin in the final, leading you to the first area of the prototype. Confused?
Although fourth in the final, Trap Tower comes next, and survives more or less intact between the two. Aside from Robotnik's words of death being shorter in the final, the only change is the addition of shining blocks in the wall that spray rings when attacked.
The final adds a "GO!" sign once the second cog starts rolling, and has six spiked smashers after the rows of generators. The prototype has only three, and follows it up with a dastardly organisation of generators to block your path. The final has a mere two just sitting there without any chance of harming anyone.
After adding another two generators to the eight from the prototype, the proto throws a few walls in your way. The final replaces these with more goddamned non-threatening smashers (hence the walls' single, sole appearance in the final's tutorial), and adds another GO at the next cog.
The proto throws a few blocks in your way that you must shove, but the final relegates them to the side as simple ring providers if you're feeling greedy.
The ladder climbing remains unchanged, though yet again, the walls are denied a final appearance and are replaced by rings.
The final ends the stage once the cog falls through the floor, but the proto continues to show the characters on a lift and exiting into the next level.
The level kicks off with the cog busting through the wall; the proto has your character run right to the far edge of the screen, but once they become moveable again they get shoved back as not to exceed the boundaries. The final has them stop early to prevent this silliness.
I'm going to hate myself in the morning.
The final adds a GO sign at the shifting sand slope, and a second boulder that just walls off into nothingness, strangely. The electricity gates near the catapults are removed in the final.
You're rewarded with rings for running into the wall in the final. The prototype doesn't encourage such nonsense.
The destruction of the machine is just explosions in the prototype, but the final throws in some debris for good measure.
The characters are free to move around while Robotnik blabs away in the prototype, but the final makes them shake off their wetness to fill that space of time. The final adds another GO when the icicles start dropping and includes a few rings along that path.
Just to mix things up, both versions have a CATCH!! sign at the horizontal ladder climby thingy, but the final only flashes it briefly. The proto has it permanently there. The final repositions the iceberg on the second platform.
The first load of raising spikes have two rows of rings in the proto, but only one in the final. The second lot of spikes loses two in the final, and they all raise at the same time - the proto has a full wall of them, but they raise in irregular patterns.
More removals from the proto - the centre spike on the left image is removed, while the leftmost spikes on the second scene are gone in the final. And the first spike after this bit is gone too. Let's just say lots of spikes are gone and leave it at that. And just to save myself from another image, a row of rings are added to the final stretch.
The quicksand merely engulfs your character in the prototype, but a pair of claws are shown to pull them under in the final, just in case lungs full of sand wasn't bad enough.
This Badnik gets a new palette for the final.
But this one doesn't even make an appearance, being replaced by a series of balls that rise up from beneath the sand.
Towards the end you're stripped of the ability to continue, and if you die, that's it, game over pal. Completing the level gives you the message "TO BE CONTINUED" before resetting, while thefinal keeps on going, for obvious reasons. Can't end a game at the third level, yo.
For all I know, there could be a plethora of unused graphics between the two or something cool like that, but my attempts at browsing data haven't exactly gone well.