It is easy to upgrade from JoomlaStats to J4age and still keep all of the statistics you have collected!
J4age has several improvements, such as:
* Better exclusion of IPs
* More accurate presentation of countries and web browsers
* It's own extension system
* Better presentation/navigation/filtering in general and better graphs
* Much more "minor" things
+ I have submitted a few bug reports, all bugs so far have been squashed.
+ The developers reply quickly to questions.
+ The development is quick in comparison to most other Joomla extensions.
+ The upgrade system is really smart.
- Nothing so far.
If you are interested in site statistics, this is (imo) the best you can get for Joomla at the moment.
My recommendation is to upgrade to J4age if you are currently using JoomlaStats.