Just to throw my two bits in...
Would have recommended Dollar Express too but too bad that you can't assign your reservation to it. We've rented from Dollar a few times and when the main counter was long (10+ people) the express pick up was for us at most 2.
Of the few times we've booked with Dollar in LAS over the last few years, we've only had one issue where they were going to charge us an extra day because they wrote down our pick up time wrong [check your paperwork before you leave!]. However, was able to resolve pretty quickly when I indicated the pickup time was impossible because of our flight arrival time (which, in addition was delayed).
As with any rental, check your car thoroughly before you leave the rental centre. It's dark in there and it's difficult sometimes to spot damage.
I wouldn't hesitate to book from Dollar again the next time we're in Vegas.