If I am looking to have a site with members getting points for action and shopping with their points - which platform is better, Drupal or Joomla for me? why?
Joomla because it comes with more support, themes and addons etc. Dunno about the points feature though.
Always Joomla... already have the extensions for your shopping & getting points. so you can easily used it.
Joomla is extremely easy for the layman, but Drupal offers much more customization. You do need to know your programming for that though.
Drupal, it has the modules you described and it offers good SEO and security which systems as Joomla don't offer. There are also enough themes available maybe not that much as Joomla has but Joomla has a lot of themes which look similar Drupal hasn't. Beside if you want to create your own theme Drupal is also easier to use. Because Drupal offers you a lot the learning curve is a bit longer then e.g. Joomla but the end result is a much better product.