Post the actual URI, then put archive URI in comment. If it helps people (with paywalls or hug of death or whatever) people will often upvote it and it will achieve a reasonably prominent position among the comments.
People notice that. Archive links remove important info from the post, this is a pretty well-settled HN thing with lots of moderator commentary if you want an infinite supply of reasoning.
Frankly, I also think it's not right to put load on archive.org's servers when not necessary.
Also, if people use archive.org explicitly to avoid paywalls, sites are just going to block archive.org, and then we won't have an Archive (to check whether the article gets updated, for instance).
You're asking me for stats about a statistically non-existent problem you made up. How many article get changed after they get posted and this isn't brought up in the comments?
Also, sometimes articles get changed because authors respond to HN feedback. Interaction between authors and the community is one of the valuable things about the forum. There's no added value to discussing some frozen snapshot of a web page - its a web forum for talking about web links rather than some revision control system.