That is not any easy question to answer, and the best answer really depends upon what you envision for the future for your site. Joomla! 1.5 has just been released in a "stable" version, and it is a significant "re-architecting" of the Joomla! code base. Many existing components will not work, at present, with the "new and improved" Joomla!, but the new architecture is considerably more flexible, and robust, than the older 1.x code base. I think it is the future of Joomla!, but many sites will likely continue to run the last stable 1.0 release (1.0.13) for quite some time to come, and that is fine.
The "table-less" reference about 1.5 does not mean that the "tables" in mySQL have been abandoned in any way, only that the code does rely on as many (if any) HTML tables to format output, and is therefore more semantic and flexible in the output generated.
The upgrade from Joomla! 1.0.12 to 1.0.13 is not overly severe, though some 3rd party components may need tweaking. Most of these components now have an updated version for 1.0.13, but that means the upgrade must be done in "steps" to insure that all components are upgraded to the same level that the Joomla! core uses, and that can get complicated for some.
As one user on these forums already pointed out, Community Builder is one such component. If you are considering upgrading to 1.0.13 from something older than 1.0.12, there are other considerations, such as the way that item-ids are handled being changed, which can impact your content linking.
All that said, 1.0.13 and 1.5 are likely to co-exist in the Joomla! world for quite some time, as they are actually very different animals at the "core" level, and some sites that are functioning well with 1.0.13 may not upgrade at all.
I already discussed that a little in your other thread, and there are some potential issues with that.
There is no need to ask for forgiveness or be apologetic at all for asking "good questions" in these forums; that's what they are for!