The tax credits that you are entitled to take will depend on your circumstances. This [broken link removed] should give you an idea as to what you may qualify for.
With regard to your income. Assuming that you are earning €42k for 2006 and your husband earned €20k for 2006 then, on the face of it, you would be better off being jointly assessed. This is because you would be able to use up a portion of your husbands unused standard rate tax band.
To get specifics as to exactly how much better off you would be you should speak to an advisor.