They are two completely different beasts, with some degree of overlap of course, but essentially very different in scope and aims.
The novel allows for a good degree of introspection, something which a screenplay would do with narration. Good narration can really make a movie, Forrest Gump, for example, but bad narration merely repeats what the viewer can see or work out for him/herself.
A screenplay is in many ways a template for people with very different skillsets to be able visualise a story. In a novel the reader must conjure up his or her own imagery and create the world and characters the author has described based on their own particular experience and knowledge of the world.
Both media have advantages and limitations for the writer, but to say which is the more difficult to write is like who would win a fight between a tiger and a shark . In water on land would be my reply.
Check out "The Martian" by Andy Weir. It's literally verbal diarhea for 300, He just started writing and made it up as he went along. But the structure of a screenplay requires tight planning and forethought. That being said, writing a good novel is harder, because of the poetic nature of language. A true writer is not only thing about the message, but the rhythym and the cadence, aliteration and meter. This is a true art that most writers never grasp. Or even recognize.
I have written both Screenplays and Novels in my time. But, the thing I consider most is that with Screenplays, you must make them on an already made storyline and they need to make sense. Novels do not always make sense. Screenplays have to be long and need to be suited to the liking of the producer or director even. And that is not always easy. The directors I had in Theatre Classes and Clubs in school were always perfectionist, and there not much better people in reality. Many directors and producers want a 100% perfect Screenplay for a soon-to-be 100% perfect performance. The harder part is developing a plausible story that makes sense.
I'm not saying that writing a screenplay doesn't allow you to be expressive and creating but it's a easier proxies than writing a novel which also has got structure to follow but you are free to develop it in very different ways. A novel always asks for special attention because in the most of times you built a story, it's like creating a new world and screenplays take courage to bring piece of histories and happenings and I see it easier to get inspired for when a novel requires other kind of stability and patience. It's also matter of time from the world experience (of what I have read) I can say that novels ask for more time than writing a screenplay.
In screenplays you have to have structure, and you have to have it all fitting together. In a novel, you can just ease into it. You can have parts that make no sense until a different part.
Novels can go on forever.
Screenplays have to have an end.
This is why I feel it's harder, you may not understand it, because as I read it over, I barely do myself.
A screenplay is a blueprint for a film, which means you're writing something that needs to be descriptive and visual in nature. Easier said than done. Since film is a collaborative medium, you must also make sure that everyone reading your screenplay is "seeing" the same film in their mind's eye. A feature length screenplay is usually 90-120 pages long, usually one page equal to one minute of screen time, so it has to be very clear and concise. It needs to be easy to read and easy to shoot.
The structure of the novel gives the writer more freedom whereas the structure of the screenplay forces the writer to put every single word under a microscope.
Screenplays look deceptively easy, but it is their minimalistic simplicity that makes them so much harder to write.
It's far easier to write a screenplay than a novel by far. Usually screenplays have a far lower level of English that needs to be used. Movies tend to be written to the fourth grade level while novels can have a college reading level attached. Most screenplays are special effects and sounds anyhow.
I am an amateur film maker, and I write short stories. Having been on both sides I will start with the screen play. It is a bit difficult at first, but mainly due to a learning curve. With the screen play It's easier to write the dialogue, expression of the characters, and camera shots. On the other hand With short stories it takes longer describe, and get into character development. With the writing aspect you write, expand, write, expand,write,and expand. It may take longer, but that doesn't necessarily mean higher difficulty. At the end of the day it's which one that comes naturally to you. That's one of the reasons writing a novel, and a screenplay are separate jobs. The both write, but take different perspectives to suit the jobs requirements.
A novel, unlike a screenplay, must be able to not only provide excellent dialogue for its characters but must be able to produce all the necessary imagery and background just through the written word. Screenplays don't have to worry about painting that picture with words- the shooting and editing process will do that for them.
The novel format also requires a far more complex plotline and character development than does a screenplay of equivalent quality due to the nature of the medium (film doesn't have enough time to get across the complexities of a plot that would make for a good novel) and the length. It's why so much of a great novel must be cut and edited to make a decent screenplay.
The entirety of a novel must take place in the reader's head from just what the author has written. The descriptions therein must by necessity be more complicated than those meant for the screen.
Both screenplays and novels (among other forms of writing) require a lot of time and effort. There's a reason (many, actually) why writing is one of the most difficult fields to break into, and it's nowhere near as easy as people pretend. As the old saying goes, if writing is easy, you're not doing it right.
A large factor is the length - novels are permitted to be much longer than screenplays and thus can require more time and effort. There are a lot of bad screenplays out there, but there are also a lot of bad novels.
Screenplays, however, are written for a visual medium, and while novels can be turned into movies, they do not usually originate that way. Naturally, there are scenes in novels that can easily be visualized, but that is what makes novels harder. In a screenplay, the effects, settings, characters, etc can be shown. In a novel, they have to be described. A screenplay might mention them in its descriptions but never its dialogue. Novels have to describe a lot more than screenplays do precisely because the latter are made to be visualized while the former must be imagined.