I began with Aluminum, light weight great sound. Then picked up a polymer, once again light weight, but lacked real depth that I got with Aluminum. I then received a Chieftain Low D in Brass, HEAVY, but man she makes up for it in depth. In my opinion its the best. The sound quality is unmatched and when in a session there is no need to ask your session meemebers to tone it down, they will most likely ask you to do so. My choices were Aluminum. Brass, or Polymer for a Low Whislte Body…Hands-down its BRASS.
I agree. I had a few different key Aluminum Burkes that I loved for the purity, I then got a Burke Brass D. That whistle has soul!! Something about the weight of the brass really adds something. I also love the way it smells! ( 
jessiek )
I wonder how the Humphreys will be, they are brass?
lolly
Good golly if he can make a low D as good as his high D, he’ll have to quit his day job to keep up with demand.
Yes Ms. Lolly! Mr. Humphrey uses a thin walled brass tube. Wise choice Grasshopper! 
I’ve liked the sound of brass but I’m quickly finding that I like aluminum more for performance. Aluminum warms in the hands a whole lot quicker than brass and I’m performing with other musicians, I find that I’m back in tune quicker. If you play more than one instrument and you pick up a cold whistle for song - especially an opening accompaniment with a piano, you want to be in tune.
The thickness of the metal is going to have a big effect on how it sounds.