FWIW I'm 5'11" (33.75 inseam) and if I were ordering a Motobecane from BD I'd go for the 56 - based on both their height recommendation chart and the fact that I currently ride a bike with a 56 Top Tube that fits me well.

It'd be nice if they also told you the head tube length for a given size, as that would help give an indication of type of fit and whether a certain bike will work for you. ie a short head tube=geared toward a more aggressive position with deeper seat to top of bars drop, one would need to set up stem/fork with lots of spacers for a less drop; vs longer head tube = geared toward less aggressive more upright position, someone racing would be limited by the amount they could drop the bars to suit a more aggressive riding position.

As an aside those new 2014 Motobecane Le Champion CF PRO look pretty nice - may put this one on my short list

UPDATE: I wrote them asking the head tube length of a size 56, and Chris from BD wrote me back - it's 170mm if anyone is interested.