At the minute, O'sullivan would destroy Hendry, but Hendry a few years ago could've walked over O'sullivan today. However, if both of them were playing their best, I'd find it very hard to choose....
As well, I think Hendry was a lot more consistant. O'sullivan might sometimes have a match (or a tournament) where he could have (and probably would have) destroyed anyone. But Hendry was incredibly good for years and years and years. That is why I view him as the greatest player ever.