[Supraperplex]

Joomla and Drupal think of it like Python and Ruby, in the sense of: the same thing but fundamentally absolutely different. I would say Drupal is better. But it doesn't really matter, either is fine and will get the job done.

If the site is not very big, consider Wordpress. For one reason: the community and with that mainly the pulgins available. For example there are WP plugins that do the whole conversion for mobile for you or do SEO for you. It really depends on what the site should do. But Wordpress can be easily the best solution.

Now, all that is moot, to a level, because all these CMS have nice importerts/exporters, so you cannot really make a big mistake that will sink everything.

If however, the site in question is bigger a site than just "a website", consider Typo3. It's a beast. It's cool. It may not worth the effort for a small shop, but if the site has enough size, try Typo3. Beast!