The first one is the 'normal lil boy' voice that Eriol uses around Sakura-tachi when he is 'undercover' as a student. It is a sweet, light voice with a precise foreigner's accent. He prounounces each japanese word clearly and proper-like for someone who's native language is not japanese. Then he has his 'real' voice when he is back to his true mage self. This one is deep and sounds like a grown man's voice. In this voice he also speaks more fluently and naturally to his 'partners in crime'. At first it is weird to hear such a grown voice coming from a 10 yr old lookin kid, but you get used to it. It's also a constant reminder of just 'who' is really in that body of his.
2. Appearance
Not too much of a biggy, but Eriol's costume in the anime is blue and black with hints of yellow in some places. In the manga his costume is multicolored with Red, Yellow, Black and Blues. They both follow the same design though. In the manga his sun staff is two times his height while in the anime it's shorter. His eye color also seems to fluctuate, I have seen it anywhere from light-blue to purple to grey.
Physically is the color of his hair. Clamp is not very consistent with his hair color I have seen it range from purple to blue to black but usually with white highlights. In the anime his hair is a dark blue almost black, with light blue highlights.
3. Reincarnation
Already explained in the Bio. Eriol in the anime is the one and only reincarnation of Clow Reed. They also seem to lean towards Eriol actually 'being' Clow, where Yue and Keroberos don't actually remember seeing him die at all. He also tells Sakura that in his former life he was known as Clow, basically implying that he is Clow himself, reborn.
In the manga Eriol is only one half of Clow's reincarnation. The other half is Sakura's father, Fujitaka. Also it is made clear that he is not fully Clow himself but only posesses his memories and power.
4. Purpose
Also explained in the Bio. Anime Eriol only has one basic purpose. To help Sakura change the cards and to make her the most powerful Sorceress in the world. In the manga he has two purposes. Not only to help change the cards but also to make her powerful enough to split his powers between himself and her father. This completes his goal of wanting to not be the most powerful mage in the world, and passing along the torch to Sakura.
5. Quotes
In the anime Eriol never calls Syaoran "cute descendant" like in the manga, and sees him more as an obstacle to his goal while at the same time being 'useful' and 'interesting'. In the manga Eriol takes more relish in teasing/freaking out Syaoran from the sidelines. He also refers to him more affectionately.
Anime Eriol is very good at faking his motives and Tomoyo never confronts him or comments anything about it to him personally. He hardly interacts directly with her and has only commented on her beautiful voice. In the manga Eriol finds himself under Tomoyo's keen scrutiny many times, as she points out that he looks at Sakura and Syaoran with "gentle eyes". When reffering to Kaho, she says he looks at her with "the gentlest eyes of all" and he has marveled at how 'amazingly' perceptive she is calling her a 'keen observer'. He ponders to himself saying that she was not endowed with magic powers but she was given an equally powerful perceptiveness and a way of sensing people's thoughts and emotions.
In the manga, Eriol has a 'revealing' conversation with Kaho about their feelings for each other. In the anime there is no true verbal indication that he is in love with Kaho, although he is nice to her.
6. Scenes
In the anime there was no 'fever' scene. In the manga he sends Sakura another test and in the end when she collapses, he cures her fever and rebukes himself saying that if he had known she was sick he would not have made her fight that day.
In the manga Eriol leaves behind a coat with Sakura when she had her fever and Syaoran tries hard to find the owner of the coat.
Eriol watches over Fujitaka in the manga. The scene where Fujitaka meets up with Nadeshiko's dad was in the anime aswell, but in the manga Eriol is in the trees watching them, he also sees the spirit of Sakura's mother and they acknowledge each other. That part was not in the anime.
As before, the manga repeatedly makes connections between Eriol and Fujitaka, all commented on by Sakura, Syaoran and Touya. This is not seen in the anime, so that element is taken out of certain scenes like when Yue discovers Eriol for the first time, in the manga Touya sees Eriol leave and compares the two. In the anime he sees Eriol leave but only suggests that there is more to him than it seems.
The scene where Eriol passes out mysterious fortunes was not in the manga. The anime elaborated the connection between Eriol and Kaho this way when he said he knew one of the preistesses who used to work there.
The entire elevator episode was strictly in the anime, so Eriol never trapped Sakura and Syaoran in the elevator.
The manga does not have Meiling, so the whole episode with her visiting for a day, and helping the grop against Eriol's spell was an exclusive anime thing.