you can say either "요" or "여"
but, pricisely speaking, it is advisible to use "요" as "여" is not a proper word.
Language constantly changes over the years and the dawn of "여" comes originally from users using the internet aged under 20th (this is my guess). This is why it is not
commonly used for a daily life but on the internet.
"여" is also used to express cute and frendship to someone you don't know.
Again, use "여" on the internet only ^_^