I live on the dark side of the moon.

I feel it's both their past. They shared the pain and grew up together(though there was a lot of Ace's viewpoint on things with the Sabo crap and haki, which seems a little pointless now given that Ace is in fact deceased so further development on his character is a bit dumb at this point). In the end though I think it was poorly played out. Emotional, yes, Oda is a master at emotions, but in the end I find that the flash back served little use given that Ace is dead. The only real use of it seems to add drama to Luffy's character and introduce Sabo and Dadan. This is however just my opinion.