DNSimple is really great. I like how they handle the web services. NameTerrific has taken a more flexible approach - we allow customers to create their own Snippets and get TSI (pretty much like AMI). So if you have a Tumblr-like platform, it will make sense to give your customers a TSI to subscribe to (and when you change your IP addresses, all of their records get updated).

Also, NameTerrific offers Anycast DNS, which is important to many users. The Unicast DNS is based on PowerDNS, the same as DNSimple, but the backend is Redis instead of MySQL - way faster.