I'd say 30% A and 70% B :-)
A because slicing allows an image to be selectively compressed in some
software. In other words, some bits of the image (as appropriate) can be
saved as JPEGs and some bits may be better saved as GIF's.
If you're compressing the entire image as a JPG or GIF then I don't see any
advantage in slicing.
B because one of the primary benefits is to create the *perception* that the
image is downloading quicker because it renders on screen as the various
bits download instead of waiting for the entire thing to download before