The big one is better if the promo fits your books. Bigger promo means more eyes on your work. A group promo is like a giant mailing list swap. If you're in a promo with 100 authors, that's a TON of readers you'll potentially be exposed to opposed to being in a tiny promo. Whether the promo fits your work is what's important. For example, I don't do multigenre promos. Those are often terrible. It's never a good idea to promote a bunch of different genres at once. You're just wasting your time. If you write romance, then stick with romance promos especially those that fit the tropes or subgenres you write. If you write mysteries, stick with mystery promos, etc. But the multigenre ones are the worst. Your books will be exposed to everyone including many people who might not read your genre.
I run a lot of BF promos and many of mine are large. Some have been very large. Some are smaller. I'm running promos now with over 80 authors. They just started the beginning of this month and already have almost 600 views and almost 700 clicks. This promo is a month-long and we're not even halfway into the month and it's doing fantastic. So you can see the potential of readers you can reach with a large promo. Some of the large promos on BF can be as good as Bookbub in terms of results. And these are SALES promos I am talking about, not free ones. So look at the amount of sales you have the potential to get with a large promo.
