It is more efficient to use if-return-return or if-else-return?

From Chromium's style guide:

Don't use else after return:

# Bad
if (foo)
  return 1
else
  return 2

# Good
if (foo)
  return 1
return 2

return 1 if foo else 2

Since the return statement terminates the execution of the current function, the two forms are equivalent (although the second one is arguably more readable than the first).

The efficiency of both forms is comparable, the underlying machine code has to perform a jump if the if condition is false anyway.

Note that Python supports a syntax that allows you to use only one return statement in your case:

return A+1 if A > B else A-1

I personally avoid else blocks when possible. See the Anti-if Campaign

Also, they don't charge 'extra' for the line, you know :p

"Simple is better than complex" & "Readability is king"

delta = 1 if (A > B) else -1
return A + delta