I think that really depends on the style of play you are looking for. Both the .45 Adventure variations and Astounding Tales are great sets of rules, but they have a different level of game mechanics. .45 Adventure rules have more structured character profiles using a system of templates, a method of diminishing effectiveness of characters as they are wounded, and detailed mechanics for resoulving heroic action. Astounding Tales, in contrast, covers the same effects in less mechanical detail. The rules emphasize story and roleplay elements instead.
Both systems are excellent, easy to learn and play and provide for the thrilling action of the pulp genre. If you and your players like to have more detailed play and resolution then .45 Adventure is for you, but if you like something more free flowing then I would recommend Astounding Tales.
There is also a set of free rules on the Pulp Figures site called Rugged Adventure, but I am not as familiar with them. On a larger scale I am hearing a lot of good things about Triump and Tragedy and there is also a Back of Beyond suppliment to Contemptable Little Armies.