Sherrilyn Kenyon vs Cassandra Clare... Question?

I am a fan of both authors and have read their books belonging to the series mentioned but neither are anything alike ... AT ALL. Sure you get a couple of similar basic details/ideas but the actual story hasn't been copied from the other.

 

There are tons of books that have been published that have similar story lines, similar characters, or ideas that aren't copied from other books.. They appear to be a lot alike but the stories are their own. 

 

What about all the romance novels out there that have time travel vixens that are sent to the past that fall in love with the dirty cowboy or the steamy indian? Does that mean of the millions of authors out there that have written the same story using the same formula mean they've plagiarized each other?

 

So in comes my question...

Where does the line between formula, inspiration, and coincidence get crossed?

Does having a similar general idea constitute as plagiarism now?

 

 

Sherrilyn

 

vs

 

Cassandra

 

Thoughts? Opinions? Arguments?