I find the 5x5 easy to re/disassemble, no awkward small internal pieces so the whole thing stays together much easier when putting it together and once you've done two layers of it you know you're home safe.
The blasted 4x4 though, if you lose just two of those tiny pieces it can take me half an hour to put it back together xD
I once reassembled it but I screwed the wrong centre on the special edge of the core...took me four hours to put back together and I had a cold xD