both are cripled versions of higher end processors..
sempron is a cripled version of the athlon
celeron is the crippled version of the pentium
the way they cripple it is they pretty much half the L2 cache.
and since, IMHO, athlons are simply better than pentiums, semprons should be better than celerons. but.. as people have said in these forums (i believe them) is that the pentiums rely more heavily on their L2 cache than the athlons, so the reduced L2 cache hurts the celeron a lot more than the sempron. which makes the sempron even better than the celeron (or, more appropriately, makes the celeron worse than the sempron).
"Is virtue a thing remote? I wish to be virtuous, and lo! Virtue is at hand." -Confucius