This is another question that I get asked a lot and, now that there is an option to get it on Kindle, it feels reasonable to answer it.
In terms of how much money each option brings in, I get the most from somebody buying the ebook from Lulu (£3.40), and the least from someone buying the paperback from Amazon (about 50p). Everything else hovers around the middle, with a range from £1.73 to £2.70.
However, it’s all more complicated than that. Sales on Amazon make me less money, but they move me up Amazon’s ranks. (Whether that makes any appreciable difference in the real world I don’t know, but I quite like watching the numbers.) I don’t much like buying stuff from Amazon myself, but I know that it’s very convenient.
I make more from an ebook than I do from a paperback – but the existence of the paperback means that it’s far more likely that someone will pick the book up, like the look of it and order one for themselves. If somebody found it in a charity shop I’d be delighted. I am not one of those authors who gets upset by the idea that more than one person might read the same copy of the same book.
My only answer, then, is going to be ‘whatever works best for you’. What I lose on the swings I make on the roundabouts, and anyway, writing isn’t my day job, and probably won’t ever be. I’m already making more per copy than a conventionally published author (the pay-off, of course, being that I’m selling fewer of them). Buy the book in whatever way is least hassle for you, read it in whatever fashion is easiest for you, and pass it on to whomsoever you like – unless the terms and conditions attached to your ebook reader exclude that possibility, of course.