Not having done a full hands-on with a Littlebox I can't comment on its usability but I have liked the quality of recordings done with them. There doesn't seem to be any dispute with its quality:price ratio, especially considering how customizable it is, and it's highly regarded by the folks here. I've been considering adding one to my arsenal.
That being said, I've never owned but have used a MixPre a few times and it's also a great mic pre, albeit, at a much larger price tag. Like all things Sound Devices, it's built like a tank and the sound of the preamp (or lack thereof) is excellent; very clean. In addition to full-size XLR outputs there's a 1/8" out that you can patch into an M-10, R-09 or any other device that has a 1/8" stereo input. One thing that may or may not concern a potential user is that the gain pots are very sensitive. It's easy to drive the output hot if you have a heavy hand while adjusting them. By all counts, they're both very solid pieces of gear but, as acidjack stated, there's not a great advantage of a MixPre over a Littlebox other than XLR outs especially when you compare the cost of each.