Both are quick and easy to change/remove the lense, you can replace the foam on the grills, not sure about the i4s, grills come with a peak, I4s come standard with a thermal lense, grillz come with a thermal coated single lense, which are good till the coating wears. think thermal lenses for both cost around the same £20 mark.
As said above its personal preference, its you who is going to have them on your head all day, you need to try them both on, see which suits the shape of your bonce.
Personally im a big fan of the grills, but i have a big head haha