I agree completely, with Studios you can do the whole park in half a day (With a fast pass that they give) and be done and kinda bored. So if you choose Universal, go for Studios first and then go to Islands.
Mythos is very good too! If you can make a reservation, go for it!
Also, don't forget that if you want to get into Potters Play Place you'll need to get a ticket in the square just outside the entrance to the Wizarding World. There you can get a return time to get right in and stay as long as you'd like!
Either way you choose you'll have a great time!