[Exim] Local or virtual?

Here is a little scenario that I would like some insight on. I have 5
different domains, each with 10 different users. I create all 50 users as
local users on my box. Which would be better, to set each domain as a local
domain and use alias files, or as virtual domains using alias files as well?

What exactly is the difference between multiple local domains, and virtual
domains??