As for the chambering, I would go for a .600 NE if I had a .500 NE already. That little bit extra.

As I have a .450 NE, I might consider a .577 NE myself, but still might be tempted by a .600.

One consideration, is make sure you can handle the recoil. I would guess you have experience in firing both. If I was buying for myself, and wanted a user rifle, ie one to actually use on elephants, I would try them out again (if possible) and give real consideration as to recoil recovery for the second shot. I think the .577 NE would win that battle. The .600 NE of course wins the "knock down" battle at both ends.

A .600 NE would be cool .... tough decision.