Most of my ebook sales come from the iBookstore. I don't have exact totals off the top of my head, but here's my ranking order of where people download my stories from:
My eBook sales/downloads
Oddly enough, amazon is at the bottom of the list in my case. Your downloads could be different depending on your audience. I would upload one story/eBook to experiment and see. It can't hurt to do a test run.
Hope this helps!
I make next to no sales on Lulu. I don't do any marketing and most of my sales come from Kindle and bookstores which carry books which have been distributed to them from Lulu and Ingrams.
The Lulu bookstore is good if you are able to drive traffic to your storefront on Lulu. If you can find a way to engage potential customers, and have them go to Lulu to purchase your book then this could be the most finically rewarding option. If, like me, you rely on people finding you by chance, a bigger bookstore, such as Amazon/Kindle might prove more rewarding.
I have my own site (click the banner below) with links pointed to retailers who are often famous names, such as Amazon, where people automatically think to look for books. I also publish 'Kindles' directly to Amazon.
Lulu does not actually market itself as a book seller, so any hits you will get you will have to generate, and the same goes for any retail site, really. If no one knows you have a book to sell and where from, they cannot consider buying it.
But I also do very little to promote my books. It's too much like hard work!
You are actually asking a very personal question
My sales are lulu and Amazon at the bottom. Ibooks and Inook are at the top. I sell 20 copies on Scribd last month. So that one might be a good place , we'll see.
Here's a statistic that may help explain the situation.I think I read somewhere that in Lulu's first 10 years of operation it had paid out $ 1 million to authors.Amazon Kindle is currently paying out $ 11 million a month to authors enrolled in KDP Select, let alone ordinary sales.If you can find a way to get buyers to come to Lulu, gofor it; otherwise put your efforts into letting peopleknow about your books on Kindle.Amazon Kindle engages in 'in your face' selling. You can buy a bookwith just one click, always a danger, and then they tell you'people who bought Book A also bought Book B' or 'We recommendBook XYZ for you'. It seems to work.