First off, I think the term "junk silver" is misleading and prefer calling it 90% coinage or something similar. Junk silver implies it has not real worth and I think it is fairly obvious that that is not the case when it comes to old US and foreign coins. with silver content I understand that the term comes from its value of being "junk" in the numismatics world, but I think as silver stackers we can see its true value.
I stack both 90% coins and bullion coins as well as some "generic" silver bars. I stack the coins and bullion for slightly different reasons. Of course it is all silver and the price for any of it rises and falls with the price of spot, but to me there is slightly more sentimental value in 90% stuff. On pieces I carry regularly (like the '45 walking liberty half above) I think about what that piece has been through in its 70+ year history. That of course doesn't add anything to the 0.36 troy ounces of silver in it, but it does make it just a little bit more enjoyable for me to buy and stack.
There is no doubt that bullion makes a little more sense for serious wealth preservation. You can find anything from 1 gram to 1kilo with everything in between. Its easier to buy/trade 20 10 ozt bars than 580 half dollars among dealers and brokers and there is something to be said for that. That bullion can be melted down and turned into something else far easier than the 90% that requires additional refining to make it pure.
In the end both are silver, both have more value than fiat currency, and this is just my ramblings on the subject!