Godaddy because i think they are more reliable. I have also used them and they are a company i can trust.
Namecheap for sure, lots of features, always very reliable and just a great registrar overall. If you are into domain names you might want to check out www.namepros.com as well
Beware with GoDaddy. They like to renew your subscription early. I paid for a 5 year domain and it renewed for another 5 years in about 8 months. Ive talked with other people and this is not an isolated incident.