The first question really should be why would you be training in both? Does one lack something that the other would provide you? If you look at what you are really trying to accomplish with your training the answer may come a bit more clearly to you.
I choose to mix Krav with BJJ because I wanted more emphasis on my ground game. In my experience the two make a pretty good mix because at least in my training the focus of the two didn't overlap too much.
I've got fairly limited exposure to Systema but from a "range" point of view it overlaps quite a bit with Krav... The problem I think you will face is which methodology you would want starting just outside of striking range.