A small diameter roller tip produces low side load on the valve, since it has rolling contact with the tip. But its small diameter roller also produces high hertzian stress on the tip surface, and it has multiple contact surfaces (on its OD and ID) where friction losses occur.

A plain rocker tip can have a large radius of curvature, which gives lower hertzian stress on the valve tip. But its sliding contact also produces more side load on the valve stem. It only has a single contact surface where friction losses occur.

The magnitude of each friction power loss in these respective rocker tip designs is a function of the contact frictions and the total moment that they are acting about. The plain tip has a single high friction contact acting about a large radius, small angle moment at the rocker pivot. The roller tip has one low friction rolling contact acting about a large radius, small angle moment at the rocker pivot. And a single high friction sliding contact, acting about a very small radius, large angle moment at the roller shaft.

In the end, it’s probably a wash between the two designs.