Hi,
I have given both the examinations in the same year, I was lucky that I cleared both the Boards in my first attempt and that too with distinction. But I still Believe that CBSE is far better that NIOS.
CBSE is One time opportunity in one year. While NIOS allows you to give examination twice a year In April and in October. Registrations are held between June-August.
Apart from this, if you are unable to clear any of your subject during oct/aprl examination, NIOS gives another option to you, In the form of ON DEMAND EXAMINATION. Where you can give exam according to your wish. You just have to select the date of your choice in the particular month. On every 1st of each month lines are open for candidates to register themselves.
Boards have their value, no matter wht board it is.. if you have cleared it with flying colours, and you have your knowledge as well.
Hope you find it helpful..

All the Best. . .