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Most every rental agency will give discounted rates for weekly or monthly rentals, but there really is not one that stands out as better or worse. You'll just have to shop around - prices can vary pretty wildly.
Actually the biggest potential expense would probably be with your two car plan. One way rentals almost always have drop off fees, so if you switched cars in Vegas, I suspect you'll end up paying significantly more money because you'd be paying that drop fee twice. If you stay with one car, you'll likely save yourself a few hundred dollars.