Why is an aufruf (Askenazim) before and a shabbat chatan (sefardim) after the wedding? I know there is an idea of a chaasan going to shul from the Pirkei d'Reb Eliezer 17 but how did it get split betweeen the Askenazim and Sefardim.
The reason for the aufruf, the calling to the torah before the wedding, is unknown to most people and therefore not kept properly. The Toras Hashlomim on Yore Daioh 192:4 suggests the following reason which requires some background understanding.
A woman before marriage has to go to mikva even if she has already been recently and is not yet a niddah again, because of a rule called "dam chimud" where the gemara assumes a woman who is proposed to, will become a niddah due to the excitement. The Toras Hashlomim suggests that the aufruf is designed so that the woman will feel secure that the wedding is really going ahead, because since the husband invests effort and expense in a small party he will not afterwards jilt her. This way, she will feel secure during the week that the wedding will go ahead and not come to have the issue of "dam chimud" when the wedding actually does go through. This is undesirable as some Rishonim (notably the Rambam) hold that nissuin cannot take place if she is a niddah.
So according to this reasoning, one should:
invest a significant amount of time/money/effort into the aufruf.
have the aufruf (or at least invest some of the time/money/effort) at least 7 days before the wedding (so that the women has time to purify herself before the wedding). I note that many years ago, Friday afternoon weddings were common so as to only have to make one big meal, which makes the appropriate time for the Aufruf in those days to be the Shabbat immediately prior to that Friday since on the erev shabbos of the aufruf they prepared for it.
The Biur Halacha (136) provides a list of individuals who take precedence for aliyot. Among them he lists a groom on the Shabbos before his wedding and a groom on the Shabbos after his wedding. Based on this, the standard practice is to call a groom up to the Torah on both the Shabbos before and after his wedding.
Among Ashkenazim the Shabbos before is celebrated as the Aufruf, seemingly because the Biur Halacha indicates that in order to create a real obligation for the groom to be called to the Torah the community should sing celebratory songs to the groom.
As for the Sfradic practice to celebrate the Shabbos following the wedding, this probably stems from the idea that a groom is like a king and should not be seen in public without an entourage.
For a more thorough treatment of this subject (with footnotes) see Aufruf by Rabbi Ari Enken
It's not pleasant, but I've heard one scholar indicate there are sources that the original Ashkenazic custom was to make a big announcement of the wedding in advance to make sure this fellow doesn't already have a wife out there who's unaware of what he's doing.