Hi,
Nice to know that you are moving towards Free-Libre software world.
Some differences between CalculiX and Code-aster, I could list at this moment are:
1. Code-aster has been in use in real world industrial applications in the complex field of nuclear engineering for about 35 years,
2. It has the benefit of this huge field application experience,
3. Thousands of validation cases, every new version must pass through, before it is released,
4. Huge documentation which is exceptional compared to most other opensource softwares.
(CONS: For Non-French speaking users like me, the machine translated documents are very often nightmares. Thanks to Google Translate that our lives are reasonably manageable these days).
5. Large and very supportive user community.
6. Of late, there have been quite a number of good tutorial videos becoming available in YouTube. In addition, a very good English book by one of the Code-Aster Gurus- J. P Aubry ( https [colon slash slash] framabook.org/beginning-with-code_aster/) is freely available to beginners or intermediate users.
Hope this partly addresses your query.
Sukumar