First, congrats on deciding between two great choices. In a year like this, that's really saying something.
Personally, I think Harvard Public Policy is pretty going to be the better option. Berkeley ARE is obviously very good in its areas, but its areas are actually pretty niche. The thing is that for every field where ARE is strong, HKS is probably stronger. IMO, the only area where ARE is truly competitive is development. However, Boston is like the world's development research hub, and I would definitely prefer to be in that city if I were doing dev.
I saw you mentioned that you are interested in political economy. If that is the case, I think your choice basically has to be Harvard. Their political economy group is simply unmatched. I know that there are people at Berkeley who do political economy, but few (if any?) are actually in the ARE department. Yes, you could probably get in touch with them even if you aren't in their department. But doing that can put you in a weird position where your home department might think of you as a little more of an outcast and your adopted department won't view you as really one of theirs. If you are at HKS, you just will not have any issues getting the advice and attention of top political economy professors.
The other big advantage of HKS is that it gives you a ton of options. It's Harvard. They're incredible at just about everything. So, you could switch your area of interest would likely still end up being advised by some of the best in the world. Their networks will open up a ton for you. I have to assume the stipend is significantly more as well, and there will be more opportunities to fund your research.
Congrats on the offers! I'm interested to hear what you decide!