Screenplays are much, much, much harder. Lots of scripts are optioned for low amounts of money (or sometimes no money, though I've never understood why a writer would agree to that). A much lower number are purchased. And a minuscule number are produced. Your chances of having your script purchased go up dramatically if you're already a working screenwriter and you live in LA.
Screenwriting is about rewriting. A script is purchased, and then new writers are brought in. The director has a vision. The producers have a vision. Changes are massive. The money for writers in Hollywood is NOT in producing original screenplays. Instead, it's in being hired to rewrite purchased screenplays, which is why it's so critical to live in LA. You have to be an insider. You have to go to meetings and schmooze and work on projects you don't give a shit about and then get fired from the projects when the next guy brought in has a different vision.
What you don't do is write a screenplay, get an agent like you do a book agent, and then wait for the agent to sell it. That just isn't how it's done in LA, especially for new writers.
Selling a novel is infinitely easier. The market is MUCH larger, and you will have MUCH more control over the final product. Even though there's a editor and a publisher, it is still your story and vision. Best case scenario, you can write the novel and then have it optioned for film. There's a tiny, tiny, tiny chance you could work on the screenplay, but don't count on it. And don't expect creative input. And don't be surprised when your sweet coming of age story has aliens and explosions added to it.