Manga
In the manga, Shigeru and Satoshi are more friendly rivals, friends that irritate each other as jokes. Overall, it seems that they are still friends and not the vicious enemies they are in the anime.

Shigeru is better looking, has a better personality, and no stupid cheerleaders. Instead of the shiny car, he has this cool bike (that looks like something out of Jet Moto). When I say better personality, it's true. Shigeru is much more supportive of Satoshi, and he doesn't tease him as much as in the anime.

Anime
I really dislike Shigeru in the anime. For one, he looks bad (better than Satoshi, though) sounds bad, and acts bad. You can literally imitate his voice by pinching your nose and talking. Literally. I'm not joking.

He's a real jerk in the anime, I'll admit that. Whenever he shows up, I can guarantee you that he'll call Satoshi a loser. *sighs shrooms*

Overall
Overall, go for the manga. It's much more enjoyable, Shigeru is a lot more enjoyable, and it's also better illustrated. The plot goes much swifter, but not so fast that you can't keep up. You can buy it at Animenation.