HD vs. flash memory.... you are generally paying more per unit of storage with the flash devices. This may be an acceptable tradeoff depending on use... flash memory is solid state and essentially immune to being jostled around. While you won't have to leave a hard drive unit sitting on your mantel for fear of upsetting it, the chances of hard drive failure are higher than memory failure. (My 40GB ipod's drive died about 13 months after I got it... it's currently out being fixed courtesy of the Best Buy service plan I bought on it.)
Also, it's ipod vs. the rest. ipod is really the fashion leader (although after digging around I decided it was the best bet for me, and I'm as unfashionable as they come), so if your kid has his heart set on an ipod, he might rather have a cheap ipod shuffle than a Creative unit with more capacity.
No matter what brand you buy, you will be able to copy existing CDs to it, and/or buy music online. I still find my best bang for the buck is to buy CDs, used wherever possible, but then, your son probably doesn't listen to the same music I do!
