The difference between current 1.0.12 and the forthcoming 1.5 Beta-2 are not easy to describe, but in essence Joomla! 1.0.x was based upon the Mambo codebase, whereas Joomla! 1.5 has been completely rewritten from scratch. Joomla! 1.0.x was written with “freedom of use” in mind; with Joomla! 1.5 expanded on this and added “ease of use”. This all sounds very nice, but what does it mean? Freedom of use refers to the unlimited possibilities you have when building web-sites, and with ease of use have tried to make life simpler for the end user, the web administrator and of course the third party developers.
Future Prospects:
Very bright future for joomla 1.5. It will take while for developers to update their products and take full advantage of 1.5’s new features, but by choosing 1.5 you will have an easy upgrade path to future versions.
Components:
All the available components are compatible with the joomla 1.0.12 version. Developers are developing more extensions now that are supported by 1.5 version.
Some components of 1.0 can also be run with joomla 1.5 in legacy mode.
Templates:
No commercial and only a few free templates have been released in joomla 1.5. But in 1.0.12 version, you can find free and commercial web templates.
Why should I choose this(1.0 or 1.5) version?
If time is not on your side, go with 1.0.12. Also, if you have a small site, the migration to 1.5 should be relatively painless.
If your project is not urgent, or if you’re building a large, fully-featured site, its well-worth waiting the extra time to take advantage of 1.5’s increased potential.
Secure:
Version 1.5 is more secure and has a nicer and more organized backend
“If you are building a new site and don’t have many extensions, make sure it will be future proof and use Joomla 1.5.”