SEOmoz and HubSpot both focus on SEO for websites and have tools available to monitor optimization and performance.
HubSpot has CMS capabilities along with lead management tracking and reporting, which differs from SEOmoz approach.
They compliment each other in many ways, however, ultimately have different target audiences in general. SEOmoz tends to cater towards SEOs and technical individuals while HubSpot is more general and targeting the less tech savvy, but motivated, small business owner.
SEOmoz is a deeper set of tools in one particular area, and HubSpot is a broad, integrated system. Both have SEO tools in them (SEOmoz is basically all SEO, after all), but HubSpot offers much more – A lead tracking and management system, analytics platform, social media monitoring tools, blogging and CMS software, and lead nurturing and email marketing tools in addition to the SEO tools.
SEOmoz’s tools are tightly focused around SEO and the SEO community in the world, but don’t touch any of the other above areas that I described. The SEOmoz tools go into more depth than HubSpot’s SEO tools, and they also offer an excellent browser extension that has both “free” and “paid” editions with varying levels of functionality.
HubSpot also includes either limited engagement or ongoing consulting services as part of the HubSpot subscription. As far as I am aware (as someone who’s familiar with SEOmoz and has in the past been a subscriber to their Pro account), SEOmoz no longer offers any consulting services. They did back in the mid-2000’s, but if they still do I’m not aware of it.
(Full disclosure: As noted in my profile, I work at HubSpot on the consulting team. My knowledge of the systems is from my work experience and having been an SEOmoz subscriber.)