Startup package, for assistant professor in USA: should I ask for a an extra PhD or an extra post-doc?
There are a number of factors to consider.

PhD students take an indeterminate amount of time to finish. Committing yourself to getting funding to cover the last "few" years for 2 or 3 PhD students is a stress you may not want your first year.

The cost to a department of a PhD student and Post doc can be vastly different. In some cases a PhD student means allocating a student from a general pool directly to you, such that there is no change in the schools budget. In other cases the stipend, tutuion, and fees all show up on the school budget. In other cases the number of PhD students may be limited (e.g., the department may not attract lots of good students) in which case a PhD student is way more "expensive" then a post doc. You ideally want to ask for whatever is cheapest (which includes money and political capital), for the department.

Another thing to consider is what is typical for a startup package. Also what can you justify. Often startup package negotiations are between the department chair and the dean. While it may seem like you are negotiatin with the chair, you are often equipping them with the information to battle the dean on your behalf.