1and1 is actually a bit cheaper, but they can take several hours to get you all set up. Then, it can take a few hours after that if you do a nameserver change to point the domain to your hosting server.
With Namecheap, it has been really fast! I've set up 4 domains in the past few days and each time the entire process (ordering + changing nameservers to my host location) took about 20 minutes total. In other words, I registered a new domain, created the add-on domain at my host, pointed the Namecheap DNS to my hosting server, and it was accessible within 20 minutes. That's really fast!
Well worth the extra buck or so I spend to get a new domain each time. So, if you're looking for recommendations on domain registrars, take it from someone who has been in this game for years now - use Namecheap. Speed matters, and you don't want to have to wait around for a slower registrar to get everything jamming for you.