Member Only Access

I am creating a website for my company and have a few questions to post on this forum. The first being, which way would be easier, creating a subdomain for a shopping portion of my site, or having that side of the main site hidden to people who are not registered members that are logged in? My initial thoughts is that it would be easier to just create a subdomain and have it look identical to the main site, but there would be subtle differences, i.e. different content, additional content, eCommerce, etc.,but if there is an easier way to do this I am all for it. Secondly, I would like to be able to make this subdomain or portion of the website a controlled access where you have to be a registered member to view this part of the website. I know people are busy and any timely help is greatly appreciated.