Are there any geographic or familial differences btw who follows who?
Most who were "kind of chassidim" of R' Naftulche even when R' Shlomo was alive- went with his children (Ungar). Also many who learnt in the mesivta and felt connected to Rubin who had a shteller there went to Ungar. However, Bobover chassidim as a whole are passive people, and many, especially the elders felt that they shouldn't pick themselves up from a beis medrash the've been davening in for years. In addition, R' Naftulche wasn't well even when R' Shlomo was living and after R'S died many did not feel any connection with R'N so when R'N died they did not feel the rabbiste should go to his daughters' husband.
As a side note, for a reason I do not know, most of the Halberstam family (aside R'BT) sides along R'MD.