Each one has its advantages and disadvantages. Though, I think in the end, they have different purposes at their heart - I usually envision EJBs as components that are part of a wholly
Java application's framework, while I'd use web serivces in places where there's a need for language-interconnection (such as .Net to Java) and single-fire operations.
The one place I could imagine this, as I illustrated above, coming into an 'either one might work' situation would be using a web service in lieu of a stateless session bean depending on the operation. This'd be in the sense of a fire-and-forget operation, but it would work written either way.
