Osage: If you’ve learned a tune on a high-D, The same fingering sequence will work on a low-D just fine. The tune will be in the same Key, just an octave lower. It’s one piece of “the magic of whistles” !
Thanks Trill. I had a hunch that was the case, but I thought I’d put my plea to the universe, as it were! 
You can (as explained by the previous poster) but the flavour of the low whistle is quite different. I prefer playing fast stuff like jigs & reels on the high whistles, and slow airs on the low ones. I find the fast tunes technically harder on the low whistle, too: running out of air and all that. Of course you can always slow the fast ones down, which changes them completely.
Thanks Anyanka. Sounds like good advice. Maybe a low whistle is the way to go - someday. At this point in my life and at my low skill level I don’t see getting near any reels except for those on my fishing poles! 
At the NAMM show in February I met the man himself and got a chance to extensively play Tony’s prototype conical plastic Low D whistle. (Not to be confused with any whistles Tony has made in the past. This one is new.)
If they’re in production now, and if the production models play like that prototype did, that would be my choice for a $100 Low D whistle.
The whistle had a very special sound and more importantly a great “action” or voicing. I switched back and forth between it and my beloved MK (in my opinion the best Low D bar none) and there was the distinct feeling of switching between types of instruments, rather than switching between two Low D whistles. I was very impressed!
Not as loud as the MK, but very special.
Now this prototype was one piece, but Tony said it will be available in a tunable two piece model.
Ooommmm. What is the sound of one whistle clicking?
Best wishes.
Steve
There are plenty of reels that benefit from a slower speed…more than I had realised. No need for speed.
No need for speed.
Hey Phill, thanks! I bookmarked the link and will delve into it.
I just “discovered” the Sessions website a few days ago
I read all the above posts, and no one yet has mentioned the Kerry Low D, for £50, made by Phil Hardy. It’s aluminum with a plastic fipple, and I have had no trouble at all getting beautiful sound out of it.
I have had to send back my Susato F whistle for repair, because the end of the top half of the whistle cracked at the joint. I’ve had no problems with any of my metal whistles, so I would say for basic sturdiness I would choose metal over plastic.
Kristen