We believe that if you're looking to produce content on a regular basis and are doing so with SEO in mind, that a sub-directory is the right route to go. Building content on a sub-directory builds authority to it's sub-domain. That is to say, if you create your blog at imoddigital.com/blog/, any value created from this goes nicely into the imoddigital.com domain. Furthermore, having it as a sub-directory jump starts the indexing of the blog because it inherits value from imoddigital.com
Should you want to launch your blog on a sub-domain, such as blog.imoddigital.com, it is our opinion that it's the same as getting a whole new domain such as imoddigitalblog.com - in both cases you're starting from a fresh beginning. Sure, there's brand leverage but that's getting too technical for this post, so let's stick with a fresh start. There are instances where you might want to use a subdomain, that may be for a piece of specific content that isn't going to be updated often and thus might become redundant - rather have it in a fresh location than hanging around on a sub-directory.
We have tested both scenarios over the years and although Matt Cutts stated that either is fine (which is true because that's a very open ended comment), the sub-directory approach works quicker and builds overall value quicker than the sub-domain approach for a content production process.
HOWEVER What's more important than deciding whether it's best to go for a sub-directory or a sub-domain is actually deciding to publish content. We've watched too many company's getting held up by decisions like this - content that isn't published is content that doesn't exist. Until you click the publish button, you've achieved nothing in the world of SEO. Dharmesh Shah, founder of HubSpot, created a fantastic flow chart that takes everything we've discussed above and lays out the process visually. You'll see that the end result is not about sub-directories versus sub-domains, but rather about moving into first gear and publishing. There is no point spending time debating sub-directories and sub-domains, rather start writing and start publishing. If you're able to set things up on a sub-directory you'll be doing the most optimal thing, but if you have a problem with sub-directories because of the content management system you use or because of the IT team, go with a sub-domain and just start publishing.
tldr; Start publishing content. If you can do so on a sub-directory.. bonus. If not, it's not the end of the world.