Ganguly, absolutely no question. Hayden was actually an absolutely brilliant ODI opener for a year or so (which was far more than he ever was in Tests), starting from that tour of New Zealand in 2006/07. Ganguly, on the other hand, was such a thing for 6 years
1996-2002. He then lost his place at the top of the order as Tendulkar insisted on batting there and Sehwag was retained for 6 years of utter uselessness based on 1 year of brilliance.
Even plenty of Australians didn't think that much of Hayden the ODI player until that tour. He'd had the odd good spell (2002/03) but mostly had done precious little of note.
Hayden's average is so high because, like Paul Collingwood, when he was hot he was so sensationally hot it disguised the fact that when he was moderate he was actually moderate for a very long time indeed.
Though clearly Hayden > Collingwood as when he was good he was better, for longer.
On the relatively rare occasions Ganguly got to open again after 2002, he
was nowhere near the force he had been.
Obviously we'll never know how Hayden would've fared had he batted three and four (or even six and seven) because he virtually never had to do it.