godaddy has better prices, slightly. and a wider selection of TLDs.
dynadot has a cleaner, simpler, less annoying interface.
godaddy may have a slightly more powerful interface, it's hard to tell, since I haven't had to do much with the domains at dynadot since setting them up.
If you want to transfer a domain from one to the other, both will make you pay to renew it. I wonder, if I were to call and say "hi, I want to give YOU all of my business, but I don't want to pay to 'renew' these domains that I've /just registered/, I just want to transfer them over to you now, and close out my account with them", whether I would get anywhere. Probably not.
I need to start registering more domains. There are people out there with thousands. I'm lacking some serious, easy, and relatively-cheap geek cred here.
edit: For example, I'm sure many of you would prefer to type 'invitotron.com' instead of 'invitotron.epiglyph.com'
edit: oh, hey. they /aren't/ cheaper anymore. .com/net/org are $8.95 at gd vs $7.99 at dd. hm.