There are many variables as others have said.
Is it even really down to these two candidates?
Should the field be expanded to include some of the coaches from the games formative years?
Anyway, taking it down to them two, there are similarities, Bennett had the star studded Broncos teams of the early 90's in parallel to Gibsons Eels in the 80's who were arguably the co-team of the decade along with Canterbury in direct opposition to the Eels.
I will say Bennett and I will justify myself along these lines:
Longevity
Gibson achieved sucess with Easts in 74 and 75 and then again with Parra in 81,82,83.
5 titles in pretty much 10 years is nothing to be sneezed at, not taking into account he took Saints to the decider in 1971.
Like Gibson, Bennett first had to taste defeat at the senior coaching level before tasting success losing on GF day in 1987 as the co-coach of Canberra.
Following this he also had a brace of titles between 92,93,97,98 and 2000, 5 titles in 8 years, although the legitimacy of the super league one can be argued as technically it wasn't the official competition.
What has set them apart is the two Bennett has captured in the next stanza of his career, knocking off a cheating storm in 2006 and taking Saints in from the wilderness in 2010.
In the modern professional era of the game, the 19 or so seasons between Wayne's first title and his latest are a testament to longevity and professionalism.
They share other parallels, they both got pumped in state of origin to the tune of 3 zip, they both came back to win a series after - Bennett many more a series to be exact.
Perhaps it is at international level that some differences exist with Gibson never having a lot to do with it directly and Bennett having both tasted success and defeat at that level, as coach of Australia and what was later essentially coach of the Kiwi team and he wrested the world cup trophy from Australia for the first time since the 1970's.
Probably summing it up, I don't think Bennett is done yet.
Whilst "who is better" is arguable for now (barely) I think one more title should just about nuke that argument.