I've eaten at pretty much every upper level place in Las Vegas, and for steak by far the best place I've eaten is Stripsteak at the Mandalay Bay.
I thought Aqua at the B was sub par, so I wouldn't have even tried Stripsteak because both are Micheal Mina restraunts.
That said, I ate there on Saturday, and it was the best meal I've ever had. The American Kobe End Cut Ribeye was unbelievable.
It can easily top $200 per person with wine, but it's worth it.
http://www.michaelmina.net/stripsteak/index.html