Reis is a Free Safety, I believe. Collins/Butler are Strong Safeties. I'm sure there are some similarities, but I don't think you can easily switch from one to the other (especially when someone else can already fill that spot).
You are right Buzz. Reis looks to have a bit more bulk and if we can get corners to add some muscle along with speed we now have the next few years it would make a difference in the tackling.