Shall I use immutable or const while passing parameters to functions

[tcak via Digitalmars-d-learn]

I have data in memory, and I want a function to take a part of 
data for processing only. It will only read and won't change.

char[] importantData;

With Immutable,

void dataProcessor( string giveMeAllYourData ){}

dataProcessor( cast( immutable )( importantData[5 .. 14] ) );



With Const,

void dataProcessor( in char[] giveMeAllYourData ){}

dataProcessor( cast( const )( importantData[5 .. 14] ) );


(Code with const wasn't tested)

Which one is better? I didn't understand it very well in 
http://dlang.org/const3.html this page.