Which symbol should I use in the title: "|" or "-"?

It makes no difference to SEO. Just remember to put your keyword at the beginning of the title and the branding at the end. Search engines always place more importance at the beginning of a text block and you should always rank for your branded terms. Check out this resource of best practice for writing meta titles:

http://www.seomoz.org/learn-seo/title-tag

I would say that "-" is better for users. The minus/dash is a "regular" character they are more likely to have come across and know the location of the key.

Stack Exchange uses the "-" to separate the prepended tags from the actual title. This question is:

seo - Which symbol should I use in Meta Title: “|” or “-”? - Stack Exchange.

The Guardian uses "|" to separate the title from the name of the paper:

Latest news, sport and comment from the Guardian | The Guardian

Given that both sites rely on SEO it clearly doesn't matter which you use.