If your purpose for using a separate domain is to segregate reputation attached to the domain, you should use a full alternate domain rather than a subdomain. My colleagues in the antispam industry tell me that they often ignore subdomains for the purposes of domain reputation, so if you really want separate domain reputations, you should use separate domains.
One thing to keep in mind when doing this is making sure your leads don't think this alternate domain belongs to criminals who are trying to defraud them. In fact, it is a popular criminal technique to register what we call lookalike domains. A couple of large consumer brands have had to face down criminals sending email from alternate domains (if our example company is Brand and they use brand.com as their domain, criminals might use brand-online.com or brand-buy.com or brand-2013.com or whatever). While there is a lot of value to separating your domains for reputation purposes, you also want to make sure you don't cause your recipients to lose trust in your messages.
My advice would be to make the website for the new domain work with your main corporate homepage, and to mention the new domain on all your sign-up forms. Additionally, before you start using the new domain, send an introductory email to your mailing list letting them know you'll be switching your sending domains, and that they should trust the new domain and not regard it as potentially criminal. For this reason, make absolutely sure your SPF and DKIM are set up before you make the switch.