Two great masks. I've owned both and will give you my thoughts.
The i4s are smaller and lighter. I found them to be really comfortable but not protect quite as well. The quick change lense system on my i4s also wore out over a couple years time span and would occasionally not secure the lenses properly (without applying pressure to the fastener to lock it in properly). Wasn't a big deal but I like the Empire retention system a bit better.
The Events are a bigger/larger mask. The mouth piece is pretty much the most breathable of anything out there and obviously flexes for bounces. The mask protects better and still offers a high probability for bounces which is nice. I ended up retiring my i4s and the Events are my go to now. Truth be told I find the pro flex is better/more comfortable for me than either but I cannot stand to change lenses on the things.