If both have early entry, which Harry Potter should I go to first?
Easy. Diagon Alley
We were there when there was early entry for both. Doesn't really matter which you do first, but we did Diagon Alley first, as that's where Harry started
the second day we started in Hogsmeade first and then back across to Diagon Alley.
In terms of wait times I don't think it made much of a difference, the waits were low in both parks for us. And we didn't have to wait long for any of the attractions. (last week)
I like the Hogswart Express from Diagon Alley to Hogsmeade better though. So if you only have one day I'd definitely start in Diagon.
Our 2 park early entry plan starts at USF. Make sure you do Despicable Me early as well!