Avis Select & Go is not nearly as good as National's Emerald Aisle or even Hertz Ultimate Choice. Basically the way it works is that they pre-assign you a vehicle and then you can use their mobile app to choose from alternate vehicles. The problem is that what the app usually serves up as alternates is extremely limited and/or unappealing, so it's definitely a YMMV situation, but on the whole, it's a huge step down from National or Hertz. Sorry to be the bearer of bad news.