Hi,
I have first-hand experience on UPF. (Attended a master there). Firstly, their faculty is really impressive, that´s right. However, a HUGE drawback is that the "study environment" at UPF is extremely bad. For example, the library is a complete joke. They do not even have standard textbooks (Romer, Mas-Collel etc.) I have no idea why.
However, this probably doesn´t matter for a summer school.