Landing page for new product launch

I`m doing a new product launch. Actually a whole new channel. Intial outreach, because the market and demand is nascent, will be direct mail, at least initially. I want to use a custom domain for the campaign for various reasons. Is it better to build the landing pages on the custom domain? Or redirect to landing pages on the main site? What are pros and cons for SEO? I understand the customer experience issues of the redirect. I guess this is in part, a question about the current thinking about the value of micro sites. Does the conversion value outweigh any SEO harm?