How is an sbt task defined using <<= different from one defined with := that references another setting's .value?

I have the following example build.sbt that uses sbt-assembly. (My assembly.sbt and project/assembly.sbt are set up as described in the readme.)

import AssemblyKeys._

organization := "com.example"

name := "hello-sbt"

version := "1.0"

scalaVersion := "2.10.3"

val hello = taskKey[Unit]("Prints hello")

hello := println(s"hello, ${assembly.value.getName}")

val hey = taskKey[Unit]("Prints hey")

hey <<= assembly map { (asm) => println(s"hey, ${asm.getName}") }

//val hi = taskKey[Unit]("Prints hi")

//hi <<= assembly { (asm) => println(s"hi, $asm") }

Both hello and hey are functionally equivalent, and when I run either task from sbt, they run assembly first and print a message with the same filename. Is there a meaningful difference between the two? (It seems like the definition of hello is "slightly magical", since the dependency on assembly is only implied there, not explicit.)

Lastly, I'm trying to understand why hey needs the map call. Obviously it results in a different object getting passed into asm, but I'm not quite sure how to fix this type error in the definition of hi:

sbt-hello/build.sbt:21: error: type mismatch;
 found   : Unit
 required: sbt.Task[Unit]
hi <<= assembly { (asm) => println(s"hi, $asm") }
                                  ^
[error] Type error in expression

It looks like assembly here is a [sbt.TaskKey[java.io.File]][2] but I don't see a map method defined there, so I can't quite figure out what's happening in the type of hey above.