Here’s the thing. No one looks at the stuff in your sidebar.
Sure, if she’s a loyal reader and she’s got nothing better to do and she’s looking for more of your great blog posts to read…maybe. But she’s already on your mailing list anyway.
You want to capture the guy who just landed on your blog after following a random link on some other website. He won’t be able to find you again if he tries, so you need to entice him to subscribe right now.
And that subtle sidebar form isn’t going to cut it. This job calls for a popup.