Pipes or colons in HTML title tags?
I would think neither would make a difference except for preference. Search engines are looking at word boundries (programming term) when parsing a string and would not recognize these characters as either a word nor a part of an HTML tag and likely will ignore them completely. From an SEO perspective, they would likely be totally ignored.
[Update]
It seems that Google is taking title tags with pipes [|] and taking it as a keyword list. This is a fairly new development likely in response to the use of pipes [|] and how they are used. It is not clear if this happens every time however. If you put your domain name (with or without a TLD) as the last element, Google will brand the title with your domain name and a colon [:] in the beginning of the title and remove it as the last element. For example: a title tag of cakes | chocolate | bunt | example should appear as example.com: cakes | chocolate | bunt. You will need to experiment with this. I have an experiment going on now and it is too early to post the results here. But this is well known so far. This does not seem to apply to colons [:] and commas [,].
If it’s only colon vs. pipe: Use the pipe.
1. The colon might (more) often be part of the page title.
It might be confusing to have two colons. Example for an article called "Top 10: Songs":
Example.com: Top 10: Songs
It seems as if "Top 10" would be some kind of second-level category here.
2. The site title should come after the page title. The colon wouldn’t make sense anymore.
As often only the first part of the title is shown to users (e.g., as part of the tab label in browsers) or users only read the first part (e.g., in search result lists), it’s more useful to see the beginning of the article/page than the site. Otherwise users wouldn’t be able to find the page they are looking for when they have several pages of your site open:
[Exampl…] [Exampl…] [Exampl…] [Exampl…] [Exampl…]
So if you agree to switch the positions, then it wouldn’t make sense anymore to use a colon:
Top 10 list of stuff: Example.com
So it should be:
Top 10 list of stuff | Example.com