3
Short answer: depends what you need to do.
Many modules are not being developed, which means not updated to the 2.3 standards and will not work. Themes in themselves should be easy to fix, as I have not had problems using my 2.0 themes on 2.3.
Like as been said before. Use 2.3 if the additional functionalities are ones you need versus 2.0.
I personnaly have converted my sites to 2.3. (one left to do
) and I don't want to use anything else.
Again the main part of your decision is are the modules you need available.
Cheers.
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