In theory, the only difference between them is that @import is the CSS mechanism to include a style sheet and <link> the HTML mechanism. However, browsers handle them differently, giving <link> a clear advantage in terms of performance.
Steve Souders wrote an extensive blog post comparing the impact of both <link> and @import (and all sorts of combinations of them) called “don’t use @import”. That title pretty much speaks for itself.
Yahoo! also mentions it as one of their performance best practices (co-authored by Steve Souders): Choose <link> over @import
Also, using the <link> tag allows you to define “preferred” and alternate stylesheets. You can’t do that with @import.