I emailed the MPP admissions about the Master's thesis any they told me that "Every student has a project course in the second half of their second semester related to their specialization which is an extended study on a specific policy topic. This study could be extended into a thesis, though this is not required." I am unsure how I should feel about this response. If I was applying to a PhD program afterwards, I would need to have started the thesis by that fall.
Hopkins has a very small MPP class of around 30 which could increase this year. Maryland has a large MPP class of roughly 115. Duke has an MPP class of 55. I think this could be a major factor to consider. However, Maryland's large MPP class is not as concerning because I would have an assistantship. On the other hand, Hopkins small MPP class could hamper alumni connections.
Just some food for thought.