Hi, Michelle! I'm the "coworker who knows a lot more about email marketing services" that Koby mentioned to you.
I would say that both services have their pros and cons.
For Constant Contact, their editor is really easy to use. But, they only allow you to host 5 images on their service, so you might have some hassle with deleting and uploading photos. However, they have a great customer service department.
MailChimp, we found, is slightly less intuitive to use, but they are, as you say, very hip and cool. They have great reporting and some cool features you can't get anywhere else--such as this cool geolocating tool. While they don't have phone support, they do tend to answer customer questions quickly via Twitter or live chat.
As you can see, they're both good services with some slight drawbacks. I'd also suggest you check out GetResponse. They also have a great product.
Plus, with these services you can always sign up for a free trial to test them out! Thanks again for your feedback and let us know if you have any additional questions.