iContact versus Constant Contact
Both iContact and Constant Contact have free trials (Constant Contact's is much longer). http://www.email-marketing-options.com has a great comparison grid that compares the main features of both services.
In terms of differences, iContact is more suited for email blasts, in other words sending an email to a large email list. Constant Contact is more suited for tracking your emails, in other words seeing who is opening them. I would recommend signing up for both and seeing which you like better. Then, just cancel the one you don't like as much during the free trial period. Best of all, signing up for the free trial doesn't require a CC. Let me know how it goes.