Until this year (I'm intentionally slowing down), I've had anywhere from one to four books going at any time. I earn most of my living as a ghostwriter or collaborator. For me, that's important because each book is in a different stage of development.
Let's say I've just finished the proposal for a book and my agent sends it to her "hit list" of editors. What do I do then? The responses usually take weeks, sometimes as long as three months, to get a contract offered and negotiated. I don't wait but work on something else.
After I finish one full draft of another book, I send it to the author and that person may take anywhere from a week to two months to return it. On my own book projects, I try to let my full manuscripts rest for a month before I go back for a final polish.
That gives you an idea of how I work, so that I haven't allowed much empty space between projects. Not everyone works that way.
My son, John Mark, helped me see the principle when he was quite young. One dinnertime I watched as he ate one item at a time on his plate and didn't touch the next one until he had finished.
I ate a little of the salad, a few bites of corn, and maybe picked up the bread. "It all ends up the same place," I said.
He has never changed, and why should he? That's his chosen way to eat.
The principle is true with writers. Some can't focus on anything except what they're currently writing, whether it's fiction or nonfiction. They push aside any new ideas or they jot them down to consider later. "I don't want to be distracted," one writer said.
So whose way is better? That's easy to answer: Your way. Whichever is the most natural, follow that pattern.
Be the best YOU that you can.
Follow what works for you.