On one hand, there is a straightforward response -- apply to the program that you are most passionate about. If you are interested in advertising and/or consumer behaviors, then apply to Marketing programs. If you are keen in how organizations and their stakeholders work, then look at Management and/or OB programs. If you are excited by online stuff, then you can try IS programs.
However, do consider a few things. First, there is a greater interaction across fields in research (e.g., IS + marketing, or IS + OB). Hence, keep an open-mind in terms of the type of programs that you are applying for. Second, do think of the job prospects for the different fields. Certain specialization are consistency in high demand (e.g., Finance and Accountancy), whereas others go through some high-low cycles. Also, since it typically takes 4 to 5 years to complete a PhD program, what is currently a hot topic (that everyone wants to do) could very much be a hot potato (that nobody wants to be involved with) when one is on the job market.
