I pay in-state ~30k/year, and I still believe HPSP is a better deal unless you need family health insurance, or are planning on a Navy career. I was an econ major, and I've analyzed it with two different models, and HPSP wins for me in both cases.
You cannot retire 4 years earlier with HSCP, you just get 4 more years towards retirement pay scale. So you will still retire at the same time.
Also, I think you are mistaken that the new GI Bill will pay your dental school tuition. It pays a % of your states highest undergrad tuition, not graduate. It does not amount to that much. Here is the calculator to figure out what you will get:
http://www.gibill2008.org/calculator.html . You will not get book stipend or living allowance, so subtract those. If you can find something that indicates the new GI Bill will pay a % based on dental school tuition, please post it, I have not been able to find anything.