While I am going to talk about a number of differences, just know that both are tools for different purposes.
HPE OneView can only managed firmware (both online and offlie, and now HPE Drivers and branded utilities thanks to the HPE OneView 2.0 release) for Gen 8 and newer systems. Why? Because firmware management is provided by iLO 4 and Intelligent Provisioning (for offline deployments) and via HPE Smart Update Utility (aka HPSUT), which relies on iLO 4 to provide the comminication path between the HPE OneView appliance and the supported OS on the host. Under the covers, HPSUM is the delivery engine that deploys the components. It's just not visible to the administrator. The baseline is managed at the Server Profile, which is just one aspect of HPE OneView, as it manages many other policies for an HPE Infrastructure.
HPSUM can manage and report HPE firmware, drivers and utilities for all of the platforms supported by the baseline (or Service Pack for Proliant, aka SPP). It is an external service that can reside in your infrastructure, and is web based. You can also use HPSUM to create custom baselines from existing SPP's, or bundle in hotfixes or eratta updates into a new baseline. Which then that baseline can be uploaded to an HPE OneView appliances Firmware Baseline Repository.
If you plan on using both HPSUM and HPE OneView, HPSUM will not deploy updates to systems that HPE OneView is currently managing a Baseline that is associated to a Server Profile.
