I don't find Aerith at all overrated. She actually seems to me as one of the more unpopular main characters of the series. I found her character pretty likeable, but some fans tend to twist her character into a Mary-Sue because of her somewhat angelic "aura".
They're both just raging romantics. Tifa and Aeris are alot more alike than some people may think 
She does keep guard over the planet in the Lifestream.