These are actually inequivalent coffeescript lines.  You can see the difference in the compiled js:
````
if @mostRecentNode?
````
compiles to 
````
if (this.mostRecentNode != null) {}
````
while
````
if @mostRecentNode isnt undefined
````
compiles to
````
if (this.mostRecentNode !== void 0) {}
````
The latter seems to work while the former throws an error when we try to call:
````
@model.get("nodeHash")(@mostRecentNode)
````