It's really just up to your personal preferences. I never use the same registrar as I host with, no particular reason though. The benefits would be it might be a bit easier to keep track of domains/hosting if any problems were to arise, but realistically I can't think of any true pros and cons to registering and hosting with the same company vs. others, unless perhaps the hosting company is cheap and running a "free domain" special.
The advantages are that it keeps everything under one umbrella, and is fast.
Personally i use gomommy for domain purchases, and host crocodile, for hosting, since they are the cheapest for each product, and just combine the 2. It takes a few clicks and about 1 min to setup the redirection.