I would probably suggest configuring a CNAME for each client so that their pages can appear under their own domain names. It depends a bit on how you are driving traffic though. If you are doing PPC (Google Adwords, etc), you’re going to want to make sure the “destination url” and the “display url” match. If you want the display ad in your PPC to match the client’s domain name (which is a good thing to do to ensure trust from your users) your option #2 wouldn’t work.
However, for some traffic sources, one global domain name might be sufficient. The biggest issue to consider is that having the trust of your visitors knowing that the landing page they are on is owned by the company whose domain is in the URL.
So, all this being said, we are about a week away from releasing a new feature that will allow you to manage multiple domains in your account. At the moment, it’s only possible to add one at a time, but this will change very shortly. If you need more than one right away, let us know and we can help you out.
Hope that helps!