I think I choose Perez because of the swamp forest maze and the lake.
And I choose the Hollows because Perez park had that swamp forest maze
Honestly, as much as I hated the early days Hollows and how difficult it was to navigate to that mission door in the FAR right back corner of the map at level 5 (which came up way too often), I feel like that made me a much better MMO player. I learned how to gauge the range of mobs and avoid them, which you don't realize how valuable a skill this is until you are trying to help a friend new to MMOs get somewhere and watch them run through ALL the mobs. Because of this I got to the point of being able to get to Portal Corps before even having a travel power and never taking a hit.
Overall, the Hollows had a decent story, cool missions, and a wide variety of terrain to mix things up. Perez in comparison was kinda dull.