I would think that the Pay Now option would be cheaper, but it's more expensive! The total cost of rental is almost double my flight cost. Yikes. So, why the heck would I Pay Now? Is there an added guarantee that my car will be waiting for me? I'm scared of all the money this trip is costing in travel alone.
I cannot think of a great reason why you would want to do this. I suppose in theory your car might be more "guaranteed", but at a major location like Las Vegas (I'm assuming the airport?) it's highly unlikely they would run out of cars anyway.
Generally the only difference I've seen is that if you "Pay Now", the price is expressed in your home currency (if you were from outside the US), whereas otherwise it's expressed in dollars, the value of which could fluctuate slightly before your trip is complete and billed. This wouldn't worry me, but for some it might if you were on a tight budget.