Since I do consulting for both OVSM and OVSD, I have done some research about this topic, so I try to answer you here.
1. For Service Calls, you will need Interaction in OVSM, for work orders, there is a Task module for change in OVSM which is similar, but no such function for Interaction yet. For incidents, you can map OVSD to OVSM(more features). If you do not want to divide too much, you can enhance your OVSM to let Interaction have more features like OVSD does for Service Call.
2. OVSM provide a war file, which you can deploy with most popular server, Tomcat, webLogic, etc. OVSM allow you to provide almost same format and function for forms between web and full Java client, which is really nice.
Overall, OVSM is so flexible, you can do much more beyond what OVSD can provide. However, it costs you more $ for sure.
Ok, for advantage/disadvantage, it might not be a good idea to post here. If you still have interest to hear, send me an email email@example.com