THE STRONGEST SOLUTION
One of two differences between the circuits is the initial Ci value. During addition it's logical low, but it appears logical hi in the opposite case. The other difference is the B inputs being inverted. We can see the solution, but let's do the whole table thing to make sure.
Given B:input, M:mode, O:output
where M is high when subtracting:
HOLY FREAKING SHIT ITS XOR LOL
...what, did you expect the actual solution or something?