domain name system - WWW record in DNS (type CNAME) pointing to a site is recommended?

I have a reseller host the sites of my clients are hosted.

One such customer wants to keep the DNS management with them.
For this, they request the CNAME for them to do the following change in DNS:

www IN CNAME xyz.website.com

The question is, what the difference is to change the A record pointing to the IP of my server to change the CNAME (As listed above?)

Answer

There are two differences:

1. With the CNAME, people trying to access their web page will have to do an extra DNS lookup. This may result in slightly slower "first page" times. (It also adds an additional potential point of failure if they aren't good at managing their DNS.)




2. With the CNAME, they'll be able to change what IP address the www host points to without having to modify the entry in that domain.