The first thing you should do is check out the professors in each one's history department and see what their research is on. Make sure you can see yourself working on the same material as at least one faculty member. It's very tough to find yourself in a place where nobody has the connections in the fields your interested in.

As a Berkeley alum, I would recommend Berkeley haha! See the departments and what you are interested in. Our Roman history prof was really good, and we had a good Byzantine prof as well. Also, as a history major, you will have to write a senior thesis on a topic of your choosing; it is hard but really fun at the same time! If you want any questions, don't hesitate to PM me!

Oh and congratulations!

Both are excellent schools with world-class history departments. You may not get to know many history professors in either place, however, since they all tend to be very famous, very busy, and have tons of graduate students.

You should take as many kinds of history courses as you can until you find an area or theme that interests you the most.

But Berkeley is the more prestigious. It's history department is one of the best in the world, along with Harvard, Yale, Princeton, Stanford, etc.