blocks are overlapped, so in theory there shouldn't be much difference?
small blocks might give less spatial smearing (what's the image equivalent of pre-echo?).
i gotta try this filter now
[edit]
on large values of sigma, on bw=bh=16, i get a strange screen pattern. increasing the blocksize to something large fixes it.
interesting filter. it removes a lot of the subtle crap in a picture. also seems to stabilize edges somewhat. too large values of sigma cause ringing, of course.