2.5 is the last in the 2 lineup, and 3 is the latest new version.  They are quite different.  2.5 is like it's predecessors, it looks like what people are used to who have used my software before.  

JS3 is a departure from that first family, and has a new interface, which is very different, and harder to learn to use, and more powerful ultimately.  It was made to be much more capable in various ways, and is much more modern.  

The newer one, JS3, or Jyotish Studio 3, runs on both Windows XP & Vista and Macintosh OSX.

The older one, 2.5, still runs on Windows.  Macintosh users MUST use JS3.  Windows users have the option of either.  Modern Macintosh users can install Windows on their Mac’s and use 2.5 there.

To Windows users I suggest they use 2.5 if they don’t like the new, the more difficult.

JS3 on the hand is newer, more powerful, interesting features, more dazzling interface, more modern, and more buggy.  JS3 is not quite finished, but the main parts are, the core is, and for the most part you can use the program and get by just fine.  If you visit some of the more "out of the way" features, you may encounter the "not quite done" aspect.  So it's a useable amazing creature, but still getting it’s fur.

The GJ 3 version, the latest version, has BOTH.  When you buy JS3, you GET 2.5 ALSO FOR FREE.