I actually have books from both. The RPGMall ones look good, but at more than a casual glance, they do look a bit computer printer-ish. But they hold up well, and feel solid, and the ink doesn't run or anything. On par with the "average" non-WOTC quality RPG book printing, and better than the "budget" lines of companies like FFG/AEG (ie, like AEG's one word books or FFG's splatbooks).
Lulu.com's stuff, well, if not for the thing that says Lulu.com on them, you wouldn't know they were done POD. Really nice looking. About on par with the bigger (but still non-WOTC) companies "prestige" lines (as opposed to their budget lines).
So, while Lulu looks better, RPGMall is definitely acceptable.