A while back I swapped the URL of a website and submitted a change of address request in WMT, after a few days it had changed. Then after a few months, out of nowhere the old domain had come back into Googles listings and the newer domain had disappeared, despite having a 301 in place and the domain change request in WMT still in place.
After a few weeks of investigating, and numerous phone calls from our client, I had noticed that mixed up in all of that coding was a canonical tag pointing to the old domain (it was very hard to see as I had minified the coding). I changed it to the new domain and voila, the new domain was back within days.
Therefore I think the canonical tag has grown stronger over the last 3-4 months as compared to a 301. I have started to rely on it more often these days.
We should keep in mind that the canonical tag wasn't created to be used as a normal re-direct, at least not as its primary purpose. Its primary usage is to tell search engines which page in a group of copies should be treated as the canonical page. Some people also try to use it for partial protection against scrapers.
Since a cononical tag doesn't re-direct any traffic, it can never be used as that kind of a re-direct. But it can direct search engines to a selected page among a group of copies, so it might be thought of as a kind of search engine re-direct. The search engine companies have said that this is a legitimate use, and they have backed this up by programming their search algorithms to formally re-direct all backlinks to the canonical page for algorithmic processing, though possibly with a small loss of "link juice".
does anyone know if pointing a rel=canonical on domainx.com to domainy.com would be equivalent of a 301?
If possible, I'd use a 301 redirect if I could, but...
-- Keep in mind if someone has access to mod_rewrite, PHP, or many other scripting languages they can issue a "proper" 301 redirect for any page/site, and the "proper" way is definitely a better way to go, IMO.
Yes, I apologise to the OP, I was talking purely from a content point of view and not as a redirect for the domain in general. I would use both where necessary unless our clients ask otherwise.
If you had duplicate websites on those domains with no redirect then use a canonical tag to the domain (and individual pages) you want to rank for your content, otherwise use a 301 redirect if you want one domain to forward to the other.
I had run into a problem only last week which I managed to fix after a few days. After a plugin upgrade in one of our Wordpress websites I had noticed that the canonical tag had disappeared and the rankings shot down 6-7 pages. After reinserting the canonical tag they went back to where they were originally after fetching Google in WMT.
It could be a coincidence as it seems that Google are testing one or two things lately, but based on my recent problems with rankings they have boiled down to the canonical tag either being formatted wrongly or missing.
That is just my findings though and somebody else may be able to better clear up how much influence a canonical tag has in SEO, but I am using it more than ever at present.