First of all, let's get our semantics straight. A novel is a book. But a book isn't necessarily a novel. It may be either a novel or a nonfiction work. The word, "book," simply refers to some form of literature that is "long form" rather than a short story or an article, for example. As for a play, that could be either a screenplay or a stage play—the former written for film and the latter for live theatrical performances. We won't even get into other types of scripts.
Regarding the differences between a novel and a script (or play), the most obvious is length. If you take an average-length book of 300 pages and adapt it into a screenplay or stage play (there is very little difference between the two except in the directions and a sense of the audience which is either watching live or watching on film), you cut, shorten, and tighten the novel down to roughly 90 pages. If you take a script and adopt it into a book, you have a MUCH tougher job. That's because you have to add not only words and pages to reach typical novel length but also effective descriptive passages, expanded dialogue, additional action, numerous subplots, and (most importantly of all) IMAGERY!
In film, with the exception of some surrealistic, nouveau, and neo directors' works, imagery is left to the cameraman's discretion. Okay, and the editor and director play a role too, of course. But primarily, the imagery is visual.
In long fiction, the writer has to create imagery through his judicious choice of words, juxtaposition of phrases, and use of modifiers, metaphors, and similes. He has no cameraman to fall back on. If something happens between the lines (the dialogue), the writer has to create it verbally and convey that to the reader through the effective use of imagery. Of course, poor writing often substitutes rambling, mostly ineffective descriptive words for imagery (bad writers "say it" instead of "show it") because it's easier to throw words down on a piece of paper than it is to create an effective image. But we're not talking about poor writing here. We're talking the cream that rises to the top.
So, which is more complicated? Writing a novel or writing a script? Writing a novel. Far more complicated. That's why some novice writers begin their writing careers cranking out scripts, thinking that, as they gain in notoriety and writing experience, they can slip into the creative (and often more lucrative) world of novel writing. They're wrong. Most script writers never learn the nuances of long-form fiction—or the demands. They may become excellent dialogue craftsmen and therefore great at their craft of script writing, but dialogue is only one small part of an effective novel.
The comparison between novel writing and script writing is a little like learning to drive a car as opposed to learning to drive a semi-tractor trailer truck. If you learn to drive that monster 16-wheeler first and then slip behind the wheel of a subcompact, you should have little trouble adapting. But if you learn to drive a car and later jump into the driver's seat of that sprawling tractor trailer, good luck! (Let me know first, please, so I can get off the street.)
So, is it safe to assume, then, that novel writers are better writers in general than script writers? That would be a logical conclusion except for the semantics of the question. What does "better writer" mean? More complete and well-rounded? Yes. Better equipped for their particular literary niche? Not necessarily. Novel writers generally make better script writers when they decide to jump ship to that form of communication because it's easier to cut to the core of a story than it is to expand beyond that story's basic plot; script writers rarely become great novel writers when they make the switchover because they simply haven't yet learned the craft of creating long-form fiction with all of its additional demands.
On the other hand, some of the best scripts in the world have come from dedicated, trained script writers and not from crossover long-fiction writers. Anyone ever hear of Shakespeare? How about Euripides, Aeschylus, and Sophocles, three of the most famous ancient Greek tragedians ever to set pen to papyrus?
Are there exceptions? Of course. But, by and large, if you're a better storyteller than a writer, stick to writing scripts. If you're a more competent wordsmith than a storyteller, get started with that novel. And let me know when it's complete. I'd like to read it.