Which is better, osCommerce or Ubercart?

The answer really depends on your particular need, including the features your business requires.

That said, given requirements that both can meet, we would recommend Ubercart 2 over osCommerce for one particular reason. Ubercart is designed so that in most cases, changes, additions and enhancements can be accomplished through add-on modules without making any changes to the original code, where this is not possible with osCommerce. The result is that having made changes to osCommerce, a new version will break those changes, and if your business changes and changes already made to osCommerce need to be removed, it's very difficult to 'unmake' those changes, especially where different changes have impacted the same parts of the software.