I do think Wintersmith is slightly weaker than Hat Full of Sky but then I actually preferred Hat Full of Sky to Wee Free Men. As the third book in the series I think Wintersmith offers more in the way of side characters (Miss Treason for example is a fabulously eccentric addition) but less in the way of the action of Wee Free Men. It also misses the "Tiffany analysis"/character delving of Hat Full of Sky. I suppose at the end of the day Wintersmith is as different from Hat Full of Sky as that was from Wee Free Men.

They all offer something slightly different.