Startup package, for assistant professor in USA: should I ask for a an extra PhD or an extra post-doc? I got an offer for a tenure-track assistant professor position in USA, Department of Engineering. The university national ranking is between 35th and 65th at the US News website. I was offered a single PhD, fully supported for 2 years (partial support for the remaining 3 years) and then I was asked how much I need for equipment. I have two questions:


Should I ask for a second PhD or a post-doc (I think for 1 year)? Some people told me to get a PhD since supervising a post-doc does not really count much for getting Tenure, while it matters a lot if one is mentoring PhDs. Other people say: get a Post-doc, he/she will be much more productive and he/she will get you papers out fast, and your mentoring effort will be on the PhD. What is your suggestion? 
I am actually also debating if I should also try to ask for a third person (in that case surely a PhD) since I am not asking too much money as for equipment (just computing cores in the supercomputer). And I need workforce since I have various good research ideas. Would I sound too greedy?


So basically, should I ask for a total of 3 persons (2 PhDs + 1 post-doc or rather 3 PhDs?) or 2 persons (2PhDs or rather 1 Phd + 1 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.