Regarding our previous discussion here (and a few posts before that), Erik J. Barzesky writes that he prefers .dmg files for archival purposes:

The time for DropStuff/Deluxe has passed. I find myself using .tgz on the command line for files I know to be safe (i.e. those without resource forks). StuffIt Expander will continue to be useful for at least a little while, but for now, .dmg is the way I intend to go.

I still use DropStuff (part of Stuffit Lite) for temporarily archiving installed applications or data files, but I agree with Erik that .dmg is the best way for archiving things.

Regarding software distribution, my experience is that most users also want to archive the original .dmg, so that’s what I use for my own products.