People often ask me what email service I use. I use Aweber. I've played with Mailchimp, but I use Aweber. Mainly because everyone I admire uses it. It doing some research here is what I found are the differences.
Aweber has better tracking of what each reader is clicking on.
Mailchimp allows complete control over designing the form (which is good as they don't have as many sign up forms – and you only get ONE per list – so no testing on what form works better)
If you have multiple lists Aweber does a better job of automating items so you don't send the same email to the same person twice (because they are on both lists).
Aweber can unsubscribe someone from your list A if they sign up for list b automatically. This save you money and time.
Both services are fine, but I use Aweber.