If I think of a book series, I think of it in terms of episodic. Each book is a season of TV shows. Most series of TV shows leave the season on a cliff hanger, unless the series is over. Book series are often written with the same characteristic in mind, that's why True Blood, and Dexter make for good TV shows, the books are episodic. One book translates into a season. Each season can essentially be watched as a stand alone, as the season conflict is usually wrapped up in a satisfying way, but something is left as a cliff hanger, something that makes you want more.
People write fan fiction and follow up books to TV series, movies (like Star Wars) because they like the characters and want to know more about what happens to them.
To me each book should have some stand alone quality. The story of that book gets resolved, even if there is a larger overall story being addressed over several books. True Blood is a character driven story, rather than an adventure driven story like Lord of The Rings. LoTR is a quest/goal story, and the characters are important as their arches are great, the goal of good vs evil is what ties the three books together. It's like two sub quests within the main quest that both get resolved on their own within the larger scheme of the story, the last part of the tale wraps it up with the ultimate fight scene and final victory that wraps up the story in a nice neat bow.
You couldn't read the Twilight books as stand alones. There is no revisiting who characters are, what they've been doing in the beginning of her stories, she just jumps right in where she left off and each book has a conclusion, but there is much left open in the overall story, which by the end of the first book you know includes turning Bella into a vampire eventually. Child being conceived was the third act twist.
Only you know the overall story of your story. How you choose to tell that story to others is completely up to you. If you can fit the whole story in one, slightly longer book, then go for it. Nothing says a book can only be 150k words, write 200k and see how it feels. If it feels too much, or like things are cramped, then it might be better to go through it and find a natural ending point for the first part of the story, where something has been concluded, break the story in two there. Depending on where your word count is, add or take away from the story to fix your word count.
You may end up with one longer great story, or two full novels. Win-win in my view. If a story has my interest I'll be more than happy to read 300,000 words. Sometimes when I'm completely in love with a story, I wish I could just wrap myself into that world for as long as possible, I find myself wishing for more...150,000 words just isn't enough. Every writer has that potential to reach, even one person, in the audience that way.