I actually use both services today so I can keep this review up to date. Overall, I think Aweber has a better deliverability rate which is a huge reason to use them. Furthermore, i like the way they do autoresponders much better than MC. On the flip side, MC has a much easier to use API if you are into coding and manipulating your lists programmatically.
i know the benchmark, you can have 3 lists, all sent from 3 different companies, and if someone opts out, they get the choice if they are listed on other lists, to tick all the lists they are on, or just some. Does aweber and mailchimp do this? I have emailed mailchimp several times with the question but no answer yet, i am pretty sure they do not, so if someone opts out it is just an opt out from that list.
Even if you've imported an existing list of contacts, you'll still want an easy way for new fans to join your email list. AWeber's sign up forms provide the perfect solution. Create a simple sign up form for your website, and if you don't have a site, we'll host it for you. Grab the link to your AWeber form and share it on Facebook, Twitter, LinkedIn, or wherever your audience is.