To me, a true victory isn't as simple as last man standing.
Especially when the side text of the next chapter questions "But how?!" or something like that. If anything at all, Sasuke won by default/survived.
It's hard to believe that Itachi sealing Orochimaru didn't take anything out of him. No way actually performing such a thing doesn't have it's drawbacks. Therefore, unless proven other wise, I do believe Orochimaru's interruption skewed the results of the fight. You can't call a clear victor in that kind of scenario, were a third party potentially tipped the scales in one opponents favour. Especially when we all saw how close(yet so far,now) Itachi was from his goal.
The MS side effects also can't be ignored, nor can they be attributed singularly to Sasuke forcing Itachi to use MS jutsus. Even though Itachi had to use them in this battle, Sasuke wasn't the origin of these side effects. Sasuke wasn't the reason Itachi choose to obtain the MS and use/abuse it's power for almost a decade. That was Itachi's own villainous thing, and many villains often contribute to their own downfall. Itachi does get some credit for kicking his own ass.
This may be a moral victory for Sasuke because he didn't take Itachi's shortcut and let the MS ruin him the way Itachi did. There's a lot of merit in that though, if it's true. Even though Sasuke's morals are still questionable, they're obviously better than Itachi's thus far.
Most importantly though, we don't know how Sasuke feels about this yet.