You can enter advertising from lots of directions. I'd suggest you apply to the division that plays to your strengths and would be most enjoyable. Cross-enrolled electives will generally be available regardless of your primary school identification.
If you're interested in a more traditional liberal arts experience, consider Weinberg Arts and Sciences. This path may appear the most tangential of options, but lots of ad people started out with english, sociology, psychology, .... degrees. Certainly the School of Communications and Medill will offer more pre-professional routes. Comm's course requirements and elective offerings will probably be the most directly "practical." Medill will be a more general hands-on media experience. Their emphasis on strengthening both writing and verbal communication skills coupled with the school's international reputation (they are even opening a Medill Qatar branch) will offer you a lot of street cred in this regard. There is a new Certificate program in Integrated Marketing Communications that will be offered preferentially but not exclusivedly to Medill students and looks like a great opportunity for those interested not in traditional journalism careers but rather marketing or advertising:
Medill - IMC Undergraduate Certificate