Mostly size - shuttle is much larger. Also I know 100% I do not need a dedicated GPU. Power should be the same between the two. Shuttle would only be better (I think?) if you needed a dedicated GPU or more ports. Btw why would you need XMBC ?
Why a NUC over a shuttle barebone? They are around $400 for barebone like these, can install XMBC on them as well.
Plus shuttle gives more options, full video card upgrade if you want, quiet CPU operation., NIC upgrade expansion. standard HD size.
Shuttle is bigger but its only showbox size, plus it looks better than these. The power savings is not noticeable vs NUC.
The only reason a NUC is good is if you are mass buying 100s-1000s at a time. But for pure media streamer/server Shuttle seems a lot better.