It also depends on the word. For example, many people (including me) would use -en for “kvinne”, but -a for “jente”. It might be possible to rank words depending on how likely people are to use -a with it. “Ku” is a word where it’s quite common to use -a.
In general it’s more common to use -a endings than to use the article “ei”. That is, everyone who says “ei”, also uses -a with the corresponding noun. Bit it’s also ok to use “en” and still use -a.