Tough question and even though both projects were originated by the same fellow (Tom Baeyens) they have take quite a different focus to cloud based process modeling.
Similarities:
- Both Activiti and Effektif are lightweight, embeddable BPMN compatible workflow engines
- Both projects are sponsored by a parent company (Alfresco in the case of Activiti, Signavio in the case of Effektif)
- Both projects are released under the Apache 2.0 license
- Both engines cite their strengths in light weight and embed ability.
- Both backends are written in Java
Differences:
- Activiti is a relatively mature project with many contributors (including myself and others from Bp3). Effektif (the engine itself) is still in Beta (or maybe even alpha release).
- Activiti (currently) uses one of a number of supported RDBMS database as the system repository where Effectif uses a noSQL backing database (mongo).
- Activiti has a very vibrant support structure through both Alfresco hosted and stackoverflow forums. Effectif support, while still good, is not as broad.
- Alfresco have and are making a significant commitment of resources to attempt to make Activiti (the enterprise edition) competitive with other commercial and open source offerings. I am not certain Signavio have the same level of commitment to Effektif (this is a personal opinion only).
Either way, in the right environment I expect both engines will be more than capable of satisfying your needs.