For me it would be Rooney.
Put peak Rooney in that Barca side and I reckon he would fit in (perhaps not perfectly) but he'd be Suarez-esque and have that aggression, dynamism to dovetail with Messi and be a decent fit in most sides and an asset rather than a hindrance.
I don't think he'd have been left out in the cold like Zlatan was and discarded so easily (if he stayed fit etc). We are assuming they are both fit and at their peak for this discussion, and Rooney for me was the better big match player at his very best.. he could score in knock out CL games whereas I don't think Zlatan had that in him.
Rooney won everything there is to win in his club career and yes he flopped with England, but he got injured during his prime and when he was fit, he was past his best for England.
Zlatan for me, individually a great talent, but always lacked that top tier dynamism to be a good fit for the worlds very best sides and thus needed teams to be built around him, problem being he wasn't good enough to then lead those sides to CL glory.. but good enough for weaker domestic leagues (Serie A wasn't quite the league it used to be during his prime there and nor was Ligue 1 a real challenge). Thats not to say Zlatan here in his prime wouldn't have been immense, he might well have been tbh but if the question is who overall had the more successful career and was arguably the better more effective player at the highest level - that answer has to be Rooney for my money.
Genuinely think if Zlatan had come to england earlier, and played for more direct sides with more pace around him, he might have had a even more stellar career in terms of europe and debate would be more in his favour.