There are a few reasons to stick with National:
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• National often has a better selection/lower mileage vehicles but YMMV depending on location and hot you hit it (luck)
• It's nice to skip having to stop at the kiosk and just get into a car and go. Sure, the kiosk is quick, but hey, it's still less convenient than skipping it.
• You can sign up for Alamo Insiders program, but there is no rewards component to the program. At least with National, you can earn points toward a free rental.

At the end of the day, in our opinion, it comes down to price. If National is only a bit more money than Alamo (or less money), then go with National. If there is a significant price differential, then Alamo is probably a better deal, but it all depends on how you value the points above and what the actual price difference is. I will say that often with discount codes and coupons, National can come in lower than Alamo in many cases, so it's always worth checking the best discounted rate for each.