I read this soon after reading Chandler's "The Long Goodbye" so comparing them was unavoidable. I think the primary thing in favor of the Chandler book is that it was more realistic. This seemed a bit more like a comic book. Along with that, it seemed that the writing in "The Glass Key" was less sophisticated. I think one place this stuck out was in the descriptions of what the characters are doing. The Hammett writing is a lot more basic in this regard. The Chandler characters seem to operating more in the real world and seem less stereotyped.
That being said, I did like this one. I didn't see the twist ending coming (although I must admit I'm generally pretty obtuse in this regard). I thought the interplay between the criminal and political and the grey area that Madvig is trying to negotiate were interesting. One interesting point is that although it a mystery novel, in many ways the mystery seems to take a back-seat to the more general maneuvering that's taking place.
I also though Ned was an interesting character. He's more a part of the seedy underworld that he deals with than Marlowe is, although he also maintains a separation of his own.
I think in the end I'm more likely to seek out more Chandler books, but I probably wouldn't turn up my nose at another Hammett