To me, the "discovering the GM's notes" sounds ... more like reading a book. (Note: I have never particularly enjoyed sandbox play--all my experiences have been roughly consistent with being told by the GM, "Go and find the fun.") It sounds less interactive and more one-sided than I have found it to be in practice, other than the most linear AP-style play.
I think "playing to discover the GM's notes" misses the idea the GM might not know everything (and probably doesn't, no matter how extensive their notes). I think there's often the implication the GM's notes include plot--as
@Fenris-77 indicates, that's the worst-case scenario (I think there's probably some disagreement about whether it's the only bad instance, but I suspect it's the case most likely to be considered bad).