When it comes to injuries, Green is more susceptible than most NBA players because just isn't that talented a player. Let's say he hurts a knee and loses 10-15% of his lateral quickness, most players have the talent to add something different to their game but it's unlikely Danny could.
I'd rather $36m/3yr guaranteed with the 4th year a team option, or at least a partial guarantee. To me Danny is the kind of player that when he loses it, he'll be out of the league. I hope it doesn't happen soon but when it does you don't wanna be stuck with him making 8 figures when that money could be better spent elsewhere.
I also think the 4th year guarantee makes a difference because it's around the time we'll be looking again in free agency. Let's say we get Aldridge this summer, that pretty much takes us out of the free agency equation for at least 2 years so it doesn't really matter what he's getting paid. If his 4th year is a team option he'd be easier to get rid of in summer 2017 as an expiring if we had to than if he had the 4th year guaranteed. Alternatively we will have more flexibility in 2018 when Parker comes off the books to get the extra cap space to find someone to add to a Kawhi/Aldridge duo if we can decline Greens option, possibly restructuring his contract or just letting him go.