One thing that one must not forget is also integration with your existing website. If your running on Joomla, WordPress, Drupal or such likes, better you find how the user integration will go with your CMS and one of those email services. You don’t want the client to have to register for your autoresponder on one form and then have to register a second time for your website! You may find that plug-ins to do such things might restrict your choice, check it out first or consult with your webmaster!
We’ve changed email marketing from iContact to MailChimp and, just now, Aweber. We changed from iContact to MailChimp to cut costs. We then switched to Aweber because Mailchimp will not allow you to start an Autoresponder series on an imported list PLUS, as you mentioned, Aweber has much more robust measuring/management tools. However, people should be aware that Aweber requires imported subscribers to opt-in all over again.
Think about implementing referral links to incentivize your subscribers to spread awareness about your brand, too. Referral links allow your current customers to promote trackable links for your business. In exchange for sending new paying customers your way, you can give them incentives like discounts, coupons, vouchers, cash, prizes or redeemable points.