MTA allows custom model loading. Using this you can make your own huge racetracks in the sky (example: Wipeout maps by Johnline). You can also use the (probably like a thousand)
client and
server scripting functions.
I just got to mention that MTA can, while SA-MP can't:
1. Sound functions (custom sounds, even songs in-game, very advanced control interface)
2. Clothes functions (custom clothing for CJ, as you've seen in single player)
3. Collision shape functions (much like checkpoints)
4. Cursor functions
5. DX drawing functions (draw your shit on the screen, even custom images)
6. Effects functions (such as add sparkling, blood, etc)
7. Engine functions (load custom models, replace cols, etc)
8. Ability to create fire
9. GUI functions (very advanced. SA-MP has dialogs, but those two can not be compared)
10. Peds (SA-MP has NPCs. Once again the two can't be compared)
11. Projectile functions (such as grenades and stuff)
12. Advanced zone (radar) functions
13. Water functions (add, control waves, change water color, etc)
14. XML access functions
MTA has more functions than SA-MP in every possible aspect. I'd bring out the custom vehicle functions for example. There are functions that allow you to get the amount of vehicle seats, turn the sirens on, change the color of vehicle headlights, set the license plate (on SA-MP doesn't work anymore), set door/light/dirt state, get vehicle occupants, and so on.
You'll be just amazed.
I currently play much in the Mini-Missions server. If you want to see some awesome race stuff, visit Mr.Green race server.