Each domain should compete on it's own merits. If you 301 redirect, the value of the domain name kind of disappears and any metric for the domain name would be hard won and keep in mind that the foundation of SEO begins with the metrics for the domain name. These day, the domain name matters less in that exact match to search terms, while it still works, weighs less. That is to say that any site that performs better for the search terms will outperform a domain with exact terms within the domain name. As well, domains must build trust metrics which is a sub-set of site metrics. Some metrics such as domain age are important for trust just to name one.
However, a site (single domain) can become muddy with content or services that are not complementary. Hence why sub-domains are sometimes used. Books, music, and videos are complementary, however, SEO and post production video may not be.
Here is what people do not know about SEO.
Google, and I assume Bing, are looking for relationships between domains. It uses current and historical registration data, citations, names, addresses, phone numbers, e-mail addresses, IP address, host information, and other data to figure out what site is related to what other site. There are two advantages of this: one, it predicts spam sites and low content quality, and two, it helps to tie brands and high quality content together to increase search satisfaction.
This means that sub-domains and domain names are tied together.
Assuming that your brand is unique enough to stand out from others and not actually ACME, there are three options.
Option 1: If the potential site topics are complementary, make the site perform where each topic has it's own sub-directory that is clear in topic.
Option 2: If the potential site topics are not complementary, make a sub-domain for each topic and tie the parent domain and sub-domains together with a link structure that is as transparent to the user as possible. This tells Google that these sites are related. Google is passing some value from the sub-domain to the parent domain.
Option 3: If the potential site topics are not complementary, make a site for each topic and let each site perform on it's own merit with a link structure that is as transparent to the user as possible. This tells Google that these sites are related.
Keep in mind that if Google determines that one related site is low quality or worse, spam, it will effect all the sites that are determined to be related.
For both options 2 and 3, each sub-domain or each domain (site) could be tuned specifically to perform well in search based upon the topic and would not be muddied or diluted by other topics.
Other things you can do:
Use mark-up for company information including name, address, phone number, e-mail, personnel, etc. This helps Google with establishing relationships.
Create a blog style or help style portion of your parent domain (assuming options 2 and 3) or site that will drive how-to or other users seeking information. Remember that giving away information does these things: keeps users coming back to your site, encourages reciprocation, encourages loyalty, encourages conversion, encourages organic links, encourages social shares.