Mystic River is better, but ...

The novel takes some time to start, but it's not really a disadvantage. Then it starts and the longer it goes, the more one sinks, the more mysteries are insollubles. Oppressive atmosphere, we feel that the outcome will be very bad.
Personally, I felt how it would end a few pages before the denouement, which could disappoint me. But it's Lehane, and despite the fact that the end can be guessed, it is carried away anyway, because that beyond the plot, it's the characters that make history.
Some readers found so that this novel is disappointing, others think that the plot is thin. I'll add that after finishing the novel, if you think about it, there are surely many inconsistencies or a lack of credibility that would not withstand a second reading.
Mystic River is better, more sustained, but denser Shutter Island is a unique journey in the throes of the human condition. So, despite his clumsiness, reading this novel's worth, and more than that yet.