What an interesting question - one really could go either way. I may slightly lean towards the Open, for a few reasons:
1. I think his trilogy of title fights against Djokovic in NYC between 2010 and 2013 is underrated relative to other great trilogies (i.e., Fed-Nadal, Becker-Edberg at Wimbledon). We all sort of look back at his 2010 Open title as though it was destiny and therefore incomparable to, say, his 2009 triumph over Fed at the AO, but really Djokovic was the one with the better resume there when they first met - a former finalist and multiple semifinalist who'd finally broken through against Federer, he was 3-0 against Nadal prior to that title fight in their matchups during the NA hard court season, 7-3 overall on the surface versus Nadal to that point. It's not like Nadal had been all that great in the weeks prior to the Open either - Fed was the one with all the momentum coming into Flushing 2010 with his annual Cincy title and a run to the finals in Toronto.
2. As well, folks complain about Nadal's draw in 2010/13, but his run in 2011 on the way to losing to Novak in the finals (one of my favorite of their matches, by the way) is rarely mentioned - he brutalized former champ Roddick in the quarters, was very strong in avenging his 2008 loss to Murray in the semis, and arrived to his match against Djokovic having won 39 out of the last 41 sets he'd contested in NYC. I don't think there's a comparable run for him in Australia, although he has had some sterling first weeks there if I recall.
3. Fewer "bad" losses at the Open once he hit his prime at both events (2008) - the Fognini match was sort of a choke by my lights (he let Fab set the mood of that match from jump street), but otherwise tight losses to Murray, Djokovic, and Pouille (who I think is a future champ in NYC) compare favorably to the Tsonga and Berdych horsewhippings and the 1R loss to an aged Verdasco (as lovely a moment as that was in a way).
4. The causal effect between his body breaking down all the time in Australia (Murray/Ferrer/Wawrinka/Cilic) and his relative inability to win quick there versus NYC (where only the Del Potro loss in 2009 was injury-impacted). Here's a stat that doesn't necessarily capture this precisely, but has some relevance: 22 bagels or breadsticks for Nadal in NYC across 8 Opens between 2008-17 (even despite missing two prime editions), versus 15 bagels or breadsticks for Nadal in Melbourne during 9 AOs in the same time frame.
Just some random thoughts-fun topic.