Financial concerns aside, the differences in the day to day job of writing include:
In a screenplay structure is everything. You have two hours to tell the story. Your forebearers have refined the the structure of those two hours, through trial and error to the extent that we know, to the minute, the ideal timings for all the major plot points. You can set your watch by some Hollywood movies, and I mean that literally.
In a novel, you do not know the pace at which your reader reads so you are relatively free to adopt your own structure (unless you are hoping to get a movie deal later). Structure is useful, but it is not everything. A beginning, middle & end is enough.
If you like describing the dewdrops on the flowers in the morning then screenwriting isn't for you. Descriptions in screenplays should be extremely succinct. A loving description of a sunset is wasted if it's overcast on the shoot day.
In novels, elaborate prose is not always appropriate, but if you must do it, there'll be an audience for it.
In a screenplay, the dialogue must crackle. You have to get your audience inside your characters heads using only the words coming out of their mouths. And they will generally be the minimum amount of words. You have one sentence to tell us JANE is angry at BILL but really she's more angry at herself. Go!
In novels, if you can't make the dialogue powerful, you have other options for getting inside the characters' heads. You can tell us their feelings, relay their inner dialogue, hop ahead or backwards to see their fears or memories without needing to indulge in flashback/forward scenes. Characters are an art and not everyone can pull them off in quite the same way.
You can write a full length screenplay in a week. Whether you should is a different matter. But it can be done if your attention span is short, or you have a story that is bursting to get out. Completing something quickly, even if it's garbage, can be wonderful motivation and reward. Once you're done, if you want to asses its structure or form a plan for redrafting it, you can read it in a little over an hour. It's easy to see the structure, hold it in your head, and fix it.
You can write a novel in a month, and thousands do every November, but it will only be around half the length of a standard novel. Similarly, re-reading, redrafting will all be slower and the intricacies will be more difficult to hold in your head the entire time. It could take a year to write a book and if the plot is complex you'll need tools to help you manage that.
An editor or publisher will want to meddle with your novel to make it read better or sell better. Aside from that, anything you want to happen in the book, can happen.
Studios, producers etc. will meddle with your screenplay, but reality will also get in the way. I've been on set when there has been some malfunction with a set, prop, location or cast member. In such circumstances the writer may get a call to redraft a scene within a couple of hours. That's not a rare occurrence in my experience (TV mainly) and that kind of compromise and collaboration is a continuous process from first hire until the shoot wraps.
Some authors do both, e.g. Michael Marshall (Smith) or Douglas Adams. If you are good at screenwriting then I would suggest you could likely pull off great novels too if you can handle the extra workload and complexity. I suspect the reverse is less true owing to the wider range of novel styles and the more flexible attitudes to structure, dialogue etc.