I'm not sure how much difference gold will make in tier discounts but Vegas hotel rates are very fluid. If you wait, you may get a better deal or the rates may jump significantly. If you're happy with the rate you see now, I'd suggest booking it. Then, if a better rate comes along later, book it and cancel the original. Just check the booking terms to make sure this is possible, but generally MGM hotel booking are fully refundable until about 72 hours before arrival. They do charge a one night deposit when you book but will refund this promptly when you cancel.