So my Lulu.com copies of the Engines & Empires books arrived today, and now that I can see both books side-by-side, I can make a direct comparison.
The clear winner here is DriveThruRPG.
Just looking at the books, the cover art is kind of dull and washed out on the DriveThru covers; on the Lulu covers, conversely, the colors pop. And that's the only area where the quality of a Lulu printing surpasses a DriveThru printing.
Lulu's binding is clearly cheaper. The interior printing is fine for the text in both books, but the illustrations on the Lulu printing don't look quite right: the grays and blacks are darker than they should be, leading to a loss of blending and transparency, and so the illustrations (and the highlighted table text) are harder to read than the DriveThru printing. The paper for the two printings is of comparable quality, but I find that I don't care for the textbook-like feel of Lulu's glossy 80# paper compared to the more tactile feel of DriveThru's 70# matte paper.
So… yeah, I'm only going to sell my books on DriveThruRPG from now on. There's little point in selling on Lulu anyway, since DriveThru is where the gamers are. I would have kept on using Lulu if the books had been significantly higher in quality, but instead, they're just… meh. I'll probably use these Lulu printings as table-copies until they're old and beat-up, while my DriveThru printings can stay on my bookshelf and look nice.
Weird to think I've been using Lulu since, like, 2008, and now I'm just gonna… not. Oh well. Onward and upward…