(I think:) Definitely give a URL if there is one, with the date you down/up-loaded the paper, and perhaps give a revision date of the paper, if it itself gives one.
AND give the more traditional reference information as well.
The URL allows people to find an e-copy, at least for a while. The conventional references do not necessarily produce copies accessible through the internet, though sometimes they do.
For the time being, these two sorts of citations give different information, have different utilities. One may take the pose that one makes the other irrelevant, but I think this is not accurate. The common "objections" to internet-accessible things, that they are "transient", while physical references are "permanent", is disingenuous, upon some thought. First, many good things are transient, which is not an argument against them! Second, physical references are equally transient, if in a different way... usually that many different libraries throughout the world maintain "cached" copies. Well, maybe Google has cached the now-gone document at a vanished URL? :)
In summary, operationally, give all the information you have in citations, even while recognizing that some of it has an expiration date.