In my experience the Staffordshire Terrier (the English dog) tends to be a bit smaller to a lot smaller and much stockier than the Pit Bull Terrier. Our Pit Bull weighs in at 65 pounds. The English Staffordshires I know tend to weigh about 40 to 45 pounds. The American Staffordshire Terrier is also a large dog like the Pit Bull Terrier but not as stocky looking since it has longer legs.
This is what one person says (and that seems to align with my experience):
“What’s the difference between an (American) Staffordshire Terrier and a Staffordshire Bull Terrier, (English) Bull Terrier or Pit Bull Terrier?
Some eight or nine varieties of dogs come within the general classification of Bull Breeds. Although all lay claim to the Bulldog as a common ancestor, there are physical differences that make each distinct from the other:
* Size – The American Staffordshire Terrier is a much larger, leggier dog – often twice the size of a Staffordshire Bull Terrier! The Bull Terrier standard does not include size restrictions and dogs from 35 to 100 pounds have been seen. However, the breed generally weighs in between 40 and 55 pounds, making it larger then the Stafford. Pit Bull Terriers also range widely in size; the early breeders preferred a smaller dog the size of a Stafford and today the American Pit Bull Terriers which are registered and exhibited seem to run on the smaller side. Others, dually registered as American Staffordshire Terriers with the AKC (Pit Bull with UKC) are larger.
* Ears – The American Staffordshire Terrier is exhibited in the United States with cropped ears, as are some Pit Bull Terriers. The ears of the Bull Terrier are naturally erect and supposed to be thin and erect. Erect(or prick) ears are a serious fault in Staffords, whose ears should be “rose” (like those of an English Bulldog) or half-pricked.
* Head – The heads of American Staffordshire Terriers, Pit Bulls and Staffordshire Bull Terriers are similar, although the cheek muscles on most Staffords seem to be more pronounced, and the head deeper through. The head of the Bull Terrier is entirely different. When viewed in profile, it resembles an egg turned on its side and is much longer than that of the Stafford. The cheeks of a Bull Terrier are not pronounced.”
( http://www.k9web.com/dog-faqs/breeds/staffords.html )